Crankcase Breathing on Post War Models

[Brian]

should anyone wish to have further details of the Breather Module ,described briefly in Members Gallery ("My 80 Plus" ) ,I would be happy to detail further.The essence of the thing is commercially available at modest cost and anyone with small turning facilities can fabricate the housing.There is no loss of orginality on associated parts(no machining of c'case etc) and its addition to a working bike is,I feel,in keeping with "an owners mod to improve the machine", as was carried out by all owners of all makes in the period when such machines were everyday transport.
Brian

Brian has provided the following graphic details of the breather system...

General Arrangement.



Larger view

Details.



Larger view

[Brian]

At last,the test-flight has taken place! The valve did its job,with no residue down the tube due to its cross section being no less than the valve-port area and that of the standpipe into the crankcase.The length of stand-pipe seems just right ,with no signs of oil squirting through from the timing chest,without a baffle. If the mist doesn't go far in the tubing,I will terminate it under the tank,instead of behind shock-absorber cover.More miles will tell.
Keep the Breather sides up.

Brian

[eddie]

Hi Brian,
               That looks a tidy bit of engineering, but I now- with my jaundiced head on- have to ask - "Why, after nearly 60 years, is this mod now necessary?" When Mr Douglas produced his first postwar model, it had a non-return valve in the breather which was discarded on later models. The Dragonfly reverted to a valved breather but was no better than the Mark series. Some 'Plus' models had a mechanical timed breather to reduce the losses caused by the pumping action within the crankcases - especially at the higher revs used when racing. The way I see it - if a postwar engine has been assembled correctly, the oil WILL stay on the inside without resorting to breather mods. The need for extra breathing means you probably have other problems which ought to be rectified first!!
           When I first joined the LDMCC (back in 1976) I fancied having a go at trialling a Comp, so set about collecting enough bits to build one. I got all sorts of advice about what I should or shouldn't do - including 'you'll need to use plenty of RTV if you want to keep the oil in!' I didn't agree, so built the engine by the tried and tested method of using paper gaskets with a smear of grease. That engine has been well used over the last 30 years - has no modification to the breathing - has been apart twice, but still has the same gaskets - and still does not leak oil. The ex works 6 days bike (rebuilt in 1998) is the same, except for a damp patch caused by a minute pinhole in one of the crankcase halves.
        I look forward to a lively debate on this subject,
                                  Regards,
                                           Eddie.

[Brian]

Hi Eddie,

Thanks for your response.I'm somewhat surprised that no one elsehas bothered thus far to comment/criticise,except a Member in Oz who built a similar Breather Valve Assembly and
likes the result.In my case it was not really a solution to an apparent problem(although the plugs were somewhat oily "as bought" with allegedly "new rings" to bed down.The oil-leaks at some joints were cured (it seems) by careful reassembly ,as you say in your experience.My decision to employ a Breather Valve of foreign origin was reasoned by wisdom from LDMCC Members who had "improved "their breathing,usualy by just venting the Filler Plug.So,it was a just in case and "if its good enough for BMW ,its good enough for me"..Now the bike is getting moving at last ,I will experiment with/without etc.I am a believer in getting the bike to at least look as if its in authentic trim,so i'd rather do without it. I value your comments Eddie ,as your reputation as a sound engineer precedes you.

Thanks again
Brian

[alwyn]

........with my jaundiced head on- have to ask - "Why, after nearly 60 years, is this mod now necessary?" When Mr Douglas produced his first postwar model, it had a non-return valve in the breather which was discarded on later models. ........

Hi Eddie,
I believe post-war Dougies imported to Australia came without non-return valves in the crankcase breathers - certainly the one I bought new c 1950 didn't have one, just a bit of ss gauze protecting the aperture internally as I recall and I clearly remember it always had a very dirty runny nose. Even when very new it passed oil from the breather in copious quantities so I don't believe it was due to "other problems which ought to be rectified first!!" as you suggest. Seems to me it was an inherent fault.

My current Dougie, a 1948 T35 (you can read something of its history HERE) also suffered the same fault leading me to modify the breathing system as described in the linked article. Although the modified breather and catch bottle has been effective enough I dislike the appearance of the bottle and drain tubes and am just about to change the system to the one adopted by Brian (and others) as described above which I think is a much tidier modification except that I question the need for such a large diameter riser tube (19 bore, close to 25 mm o/d) - after all, the original breather was but about 8 mm diameter. Would someone like to comment upon this please? 

I think oil leaking from crankcase and gearbox joints is a separate matter that more appropriately may be a subject for another topic if someone wishes to continue the discussion.

Alwyn

[Brian]

Hi Alwyn,

Re the size of the Breather Pipe.Pressure drop rather than back-pressure is the name
of the game.The tube has a bore the same cross-sectional area as the valve-opening.
This BMW 'valve is suited to a much larger crankcase volume-change than a 350 Douglas,
so its having an easy time.As it happens,nothing at all now blows from the 8mm tube that
is encastre with the Timing Cover and leads from the gauze-covered port at the top.
I will,as I have said suck it (ha ha) and see.
As far as "size matters",ask a Vincent owner how big his "elephants trunk" is ,as a good
mod on post-war twins.That comes from a whopping elbow on the timing chest.(see "Know
thy Beast")

Best wishes

Brian

[KiwiJohn]

My 80 Plus has the central air cleaner, would venting into there be a good move?  Then she can suck up those oily fumes and blow them out the back!

[trevorp]

Ok im going to throw my cap in the ring crankase ventilation was developed to rid the crankase of unburnt fuel to stop oil contamination there will always be blow by past the rings.
most of these systems were fitted to bikes as the got older and had a lot of blow by past the rings and causing the cranckcase to build up excess pressure and thereby leak oil
i agree with eddie that with an engine in good condition you should have no problem with oil leaks but if fitted they will assit to help prevent crankase contamination and this can only be a good thing

[eddie]

Trev,
          Crankcase ventilation is not necessarily the same as having a large bore breather. Some car engines have more than one breather - one of which is usually routed so that the fumes get drawn into the induction system, thus creating a circulation which draws away any unburnt residue from the fuel.
        Any petrol engine is basically a pump which has been adapted to incorporate a power stroke by adding fuel to the incoming air and detonating it. Now, if our engine has a displacement of 348cc (per revolution) above the pistons, the displacement must be the same below the pistons, thus causing pressure waves within the crankcase. An open crankcase overcomes this problem at a stroke but is totally impractical as there is no containment of the oil, etc. A totally sealed crankcase goes to the other extreme and any blow-by cannot escape, thus causing oil to be pushed back past the piston rings, and any other ill-matched parts. If we now introduce a small breather, the downward stroke of the pistons will tend to evacuate the crankcase, causing a pressure drop on the return stroke. The less dense atmosphere in the crankcase then consumes less power in pumping this air around and generating pressure waves - leaving more power to turn the back wheel. If we increase the size of the breather, we will pump a greater volume of air/fumes from the crankcase and therefore consume more power. The introduction of a nonreturn valve prevents the gasses from being sucked back into the crankcase, but it takes pressure to open the valve and if you multiply it by the ratio of the area of the valve to the area of the two pistons, then you get quite considerable pressure working against the power of the engine. Changing to a mechanical timed breather is much better, but there can still be some hidden snags. The timing of the breather may be affected by where it is sited - the pressure wave generated under the piston has to have time to travel to the breather, so the breather may need to open as the piston is rising! At low revs, this will result in air being drawn in, but as we get to the usable rev band, the pressure waves will coincide with the opening of the breather and cause a pressure drop within the crankcase. (Check out a DBD34 Gold Star -it was  probably one of the most efficient push rod singles of the 50's, but the pressure wave had to travel from the bottom of the piston, through the crankcase, into the timing case to the far end before encountering the timed valve and its escape to freedom, so the valve is timed later than would be expected).
        Modification and improvement?? of our engines is a very complicated process and if we are only going to use them as Mr Douglas originally intended, "Why, after 60 years are these mods necessary?" They are often only a remedy for a symptom and do not tackle the root cause of the problem!

                                 Eddie.

[Brian]

Hi Eddie,
A nice dissertation on crankcase gas flow. I would be interested in your view as to why BMW felt the need to incorporate the Valve I have used ,in their standard design of flat-twins. The ease with which my assembly "breathes" ( a barely audible flatulent sigh ) as you kick the engine over) seems to indicate very very low energy being absorbed.The reed passes a vey small oil-mist into the upper housing,and so far none into the long hose.I intend to experimenty with the hose area, as I use the bike, and also run it without, for comparison ,as the "new rings" bed down.

cheers

Brian

[eddie]

Hi Brian,
              There are many factors that influence the detail design of any manufacturer's products. It is always a compromise between what is best, simplest, cheapest or easiest to produce or fit, and whether it can be incorporated into other features of that particular engine. Dare I say it - Mr Douglas was probably looking at supplying one end of the market while BMW were setting their sites a little higher, so the cost element was probably less important to them.
            The breather on the T35 is in a tubular boss on the inside of the timing cover. This boss then sits inside the recessed face of the magneto idler gear which, in turn, must act like a centrifuge throwing any solids/liquids away from the breather port. As far as I know, there were three different types of breather - the first being a crude non return valve assembly that is pressed into the bore of the boss. The second was merely a steel tube with a gauze baffle - to catch any oil mist? The last version was a modification of the previous type but had a flange on the tube, presumably to assist in removing it, should the need arise.
        As Alwyn has said, his early machines always had a 'runny nose' - probably caused by oil running down the inside of the timing cover onto the boss and getting picked up by the escaping gasses. Later machines suffered  this problem to a lesser extent - probably by accident. Mr Douglas did not make a particularly good job of flanging the tube for the breather, which, when fitted, stood proud of the face of the boss, thus creating a gutter which channeled the oil around the port. Of course, having the escape route pointing directly downwards to the bottom of the timing cover didn't help matters! Also that exit point is directly in line with any water or road dirt that gets kicked up by the front wheel. When I got hold of the 6 days Douglas, I found the engine had a fair amount of water and rubbish in it - probably pulled in via the breather - this has now been routed to exit into the intake tube to be breathed in again by the engine. When I rebuilt my 'standard' Comp, I blanked off the hole in the bottom of  the timing cover and rerouted the breather pipe to go from the internal boss to exit at the top of the cover to prvent the intake of water. (It worked - in one trial I had the whole of the engine under water in a section where we were not allowed to inspect it first! - not a drop of water got into the crankcase - I wish I could say the same of the carbs!!)
        Detail changes in engine design seldom happen in isolation - they can easily be the start of a chain reaction - and each design has it's own peculiarities. What works for one engine may not necessarily be as good on another.

                      Regards,
                                        Eddie.

[trevorp]

True eddie what u describe is true for a single cylinder engine but on a flat twin as one piston moves down the bore the other piston is moving up the bore creating thus negating the pressure wave.
And as for running positive crankase ventilation through the inlet manifold this was designed to reburn any unburnt gases and not discharging them to atmosphere
The reason fo having a second vent on a car engine is so that negative pressure in the crankase will cause dust and grit to be pulled in at the weakest point seals etc  a filter is usually attached to the second vent so dust is not drawn in there
and although i love my duggie i wouldnt declare that mr douglas had his engine design worked out as an 80 and 90 plus demonstrates that some engines were more powerful than others one would ask why is one more powerfull
the mark series was severly under carburated which may have been deliberate to improve torque at low end but certaintly did not help its top end
this may have also come from the fact that these engine were basically designed from a pump or generator and put into motorcycle frames
the fact freddie dixon improved the flow on the mk3 and upwards in head design and larger pushrods among other things shows i think the big difference between the mk1 and the 3 upwards
interested on your thoughts

[Chris]

Sorry Trevorp but you must have a very strange Douglas if yours has one piston moving up the bore as the other is moving down the bore. The whole purpose of a "vibrationless" horizontal twin is that there is perfect primary balance as both pistons move out and in together. This has the effect of creating negative pressure in the crankcase as the pistons move out and positive pressure as they move in. No doubt Eddie will respond more fully in response to your posting. Chris.

[eddie]

Come on Trev, have you been out riding that one up/ one down Douglas again? Has it got your brain tingling?
Seriously though, with the passage of some 60 years, some of your observations about the Douglas have proved to be correct, but back in 1946 this planet was just recovering from World War 2 and a docile but reliable workhorse was more desirable to the average owner than a tuned racing machine, so Mr Douglas produced the T35. As you say, it was based on the wartime generator engine, and as a motorcycle engine was expected to produce about 16 to 18 horsepower - a massive increase on the 4 to 5 that it produced as a stationary engine. It soon became apparent that the high dome pistons were the cause of rough running when made to work hard - the flame front had too far to travel across the crown, so Freddie Dixon redesigned it with a shallower combustion chamber and flat top pistons. By 1948, competitive motorcycle events were being organised again, so the Mark 3 was modified to produce the Mk3 Sports - on test it proved to be the fastest production 350 of it's time. So I would say that Mr Douglas's design was not far short of 'State of the Art' - OK some of the materials used fell short of what was required, but the basic design was good. By the early 50's, Clubman's road racing was on the increase and Douglas tried to capture part of that market by producing the 80 and 90 Plus machines - which were available as an out and out racer or just a tuned road machine. As the years passed, the performance of our beloved Douglases improved - because that was what the public demanded - until the Dragonfly, that is!
        Another factor which often gets overlooked, is the fact that by the late fifties, the old pool petrol had gone and we had better quality fuel - in Britain we could buy Esso petrol with a heady 103 octane! Along with this, lubricating oil had also improved, so the improved power didn't come at the expense of engine life.
       During the 80's and 90's, here in britain, George Easton set about improving his Mark series bike for road racing, and ended up with a 500 that could stay with a Manx Norton but most of the internals were of George's own making. So, with the use of modern materials, fuels and oils, etc. the Douglas can be made to go considerably faster, but if that machine had been produced in 1946, I doubt if anyone would have wanted it or been able to afford it!
     So, I reckon Mr Douglas got the design about right (at the time) but should have settled for better quality materials.
     Keep the thoughts coming,
                          Regards,
                                         Eddie.

[trevorp]

yep ill agree slap me for the the piston movement my brain was on another planet comes from working on to many vertical twins 

[graeme]

yep ill agree slap me for the the piston movement my brain was on another planet comes from working on to many vertical twins 

I presume you mean Honda Benly style vertical twins? All British vertical twins have the pistons moving up and down together!

[trevorp]

i havent had much to do with british bikes till the mk came along mainly suzuki but i would be suprised if all british bikes had both pistons a tdc at the same time but then again ive been wrong before lol

[graeme]

I'm pretty confident I am correct here Trevor - I can't think of any British vertical twin that isn't a 180 degree engine ie both pistons rise and fall together. 

[pjondeck]

I am perhaps a little reluctant to "stick my neck out" on this issue in light of one of our colleague's recent mauling as I do not consider myself any sort of an authority so E&OE please. Alwyn has encouraged me to relate my experience with this issue so here goes. Firstly I have incorporated Brian's BMW PVC approach albeit a little modified to try to make it as unobtrusive as I could. (see photo attached). The main difference is that I have reduced the breather tube size to 12mm ID and have run it back over the tank supports and by following the curving line of the frame under the seat have hidden it as much as possible. I have then run it into my imitation battery box where it drops into a plastic bottle. As I use a smaller 6v battery in the box there is plenty of room to do this. After 400 miles I have about an eggcup of oil in the bottle. By doing this the "asthmatic wheeze" is also mostly eliminated.
As to what Mr. Douglas's approach was I have the following to offer. I have a Mk1 engine that I believe to be pretty much original. The screened bezel that was fitted in the boss in the timing case is as in the attached photo labelled "mesh breather". (Note: the breather is not pressed home in the picture, it's hard to get out!)
I also have the unit that was fitted to the MK3 which I also believe to be original. Please see the photos of the PCV valve top and bottom views and one of it installed in it's correct orientation. The PCV unit sits inside the timing case boss on the raised flange and is secured in place by the shorter, unmeshed belltube as shown.
To thoroughly clean and free up the original PCV I found it necessary to push back the three small crimp retainers on the valve and disassemble it. This did not prove to be a problem and the valve I am confident is working as good as it ever did. Alwyn has suggested that since I have the original arrangement working I should perhaps remove the BMW valve and see what happens. I may do this but have a small reservation. The engine is breathing so freely as it is and I am getting a negligible weep of oil from the vent tube in the timing case, to the extent that after 300 miles of running on the Centenary Rally at Moss Vale I had many comment to me at the operating cleanliness of the engine. If I disable the BMW device and hence cause an increase in crankcase pressure, will I create for myself leaks that don't go away when I restore the device. It seems to me that even if both devices are working to reduce crank case pressure then the small deviation from original is thus justified, (I hear you Eddie!) I have spoken to riders who race early machines about the extent to which they go to ventilate the crankcase in the quest of another horsepower or two and whilst this is not the first consideration of restorers per se, it seems to me that we can't have too much of it.
I guess that's about all I have to say about THAT!
cheers
Peter     



MK3S-engine.jpg


mesh_breather.jpg


PCV_top.JPG


PCV_bottom.JPG


PCV_bezel_and_PCV.JPG

[trevorp]

I have worked in the auto industry all my life and please dont view its as a mauling pete i encourage healthy debate as all knowledge is good knowledge. One thing is has brought to light is that flat twins may in fact need to breath more than than vertical twins and graeme may want to check out this link as we need to clarify that douglas has both pistons at tdc at the same time it seems that truimph bsa and norton have offset cranks.
One thing that it has brought to my attention is that as eddie said an engine is just basically a compressor so 350 cc of air is compressed in the crankcase as each piston goes to bdc where in vertical twins as on piston goes up the other is going down to some degree and whilst this creates a pulsing in the crankcase one is tending to cancel out the others pressure wave and only blow by creates pressure in the crankcase.
i can fully understand eddies point of view and boxer engines are among some of the most powerfull engines ever made eg porsche and subaru and i admire eddies love of the douglas marque but  i will say that engine development is a continuing process that still goes on to this day
Let the debate continue and if im wrong again i still have one cheek unslapped 

[eddie]

Trevor,
            It is about time we sorted out this myth about 1 or 2 pistons being at TDC on British bikes. As far as I am aware, virtually all British 4 stroke twins were - in standard trim - set up so that both pistons are at TDC at the same time - including Douglas, BSA, Triumph, Norton, Ariel, Velocette, Matchless and AJS. Only after they left the factories, did the tuning boys attempt to change the configuration of the cranks to give an offset (as in the link in your previous posting). Only the 2 stroke twins were set up with 180 degree cranks until the early 60's when Honda used a 180 degree crank in the Benly twins (4 stroke) and, I believe, Laverda did likewise when they produced a bigger version for their 750 twins. Back in the 60's or 70's, several people tried - with varying degrees of success - the 180 degree configuration on British twins in an attempt to eliminate some of the vibration, but standard motors had both pistons at TDC at the same time.
                                   
                             Eddie.

[graeme]

Eddie - you beat me to the reply! I'm afraid that Trevor is going to have to get both cheeks slapped!

[eddie]

Graeme,  I agree - but not too hard, he might get to enjoy it!!

                                  Eddie.

[trevorp]

ok im going to take up boxing its less painfull but at least  now we all know
now i wonder why all british bikes were percieved to leak oil could it be crankase compression ok was a joke and im hiding in argentina for a while

[trevorp]

OK now my cheeks are less red lets get back to the subject now i have a better understanding of how an engine works lol
A few things need to be sorted out and the major one will be i believe that if the pcv or flap valve is used then wont it just just suck air through the smaller tube at the bottom and this will lead to contamination of the front case as bits of debris will be thrown off the magneto gear into the crankcase
Eddies thoughts appreciated, so if u do this mod some sort of cleaner like air cleaner will need to be fitted to the small tube bottom of front cover, and this being a  restriction will effect air getting back in on the upward stroke
i am starting to see eddies point of view now as when u start one fix u can create other problems
Has anyone ever done a crankcase compression test on a running mark series as would be an interesting bit of data

[pjondeck]

Hi Trev
Not a problem if the arrangement in the timing case is as depicted in my last submission in this topic. I am using the timing case PCV as shown here anyhow and it will be closed whenever the pressure on the outside of the case exceeds the pressure on the inside.
Am I missing something?



CV_bezel_and_PCV.JPG.

[tommy]

OK, a snippet for Trevorp etc.
This is far deeper than fitting a breather. You are over the top on solution, rather like BMW.
I have BMW and some are great, others way over engineered.

Anyone ever rebuilt a Douglas engine using club/Hepolite pistons????

Did you ever find that the bike smokes like a chimney for miles after start up? Left pot more so?
Oiled plugs with whiskers on. Poor idle, hard to start.

Ever tried drilling the piston at the back of the control rings to get oil away?

These pistons are poor alloy and you have to bore out with clearance to allow for expansion.
Most of them cease.

When the bore clearance is large you will get escaping gasses past the rings into the crank cases.
As the pistons go to TDC and BDC together you get a pumping (boxer) action.
This compresses the gasses in the cases with very few places to go, apart from a small bore piddle pot tube out the front lower of the timing chest.
Maybe it can go out the mag gasket giving a leaking engine or perhaps, it will blow past the seal at the back of the crank (only felt) into the bellhousing and clutch?
Maybe down the breather?? Under the engine and onto the back tyre as Mr DOUGLAS intended?

When an engine is passing gasses past the rings like that, you have to vent the pressure?
One stroke of pumping in to the central cases will have huge PSI.

The best way is a gas pipe fitting (will look back at notes) screwed directly into the oil filler cap with a copper tube soldered into the top.
Then a big bore plastic flexi pipe straight out the top, running under the tank and seat out to the back of the rear mudguard.

You can fit this mod, but then CURE the problem faster by using a straight engine oil.
Silkolene Chatsworth 40 monograde is a good bet.

The problem is that the modern oils are so good, the rings will never bed into the bores.

Go with the above and they will be bedded in within 500 to 1,000 miles as per the handbook. In those days they were using monograde oil.


Think about it!!!

More later on modern materials etc, if you want it.

I agree with Eddie. An engine with good gaskets will not leak oil. Mine does not loose a drop and frankly I am not frightened of thrashing it or riding 500 miles on it, with no breather.

Having said that, a trials machine is generally low revving engine looking to trickle the power than high revs so I use a clear gasket sealer instead of grease.
Grease gets hot then melts.
I have used silicone as a gasket material with engines using wills rings revving to 10,000RPM Hillman Imp.
Won RAC championships with them, but don't reckon I would hold much hope with grease.
They were also trials machines, but cars.
Gordon Jackson used to try it, for those of you who remember him.


Tom

[pjondeck]

Tommy,
Once again I am no expert and am, like you, a bit of a cynic. I do try to be a bit philosophical when it comes to British motorcycles though for the sake of my sanity.
It seems to me that the objective is to maintain as low pressure in the crank case as can be managed without detracting too much from the originality of the machine. I am more focussed on the external appearance of the machine and am happy to make mods to the internals if they will improve the situation. Eg. my SS80 Brough Superior has a re-engineered bottom end with triple roller bearings, it runs Chrysler slant six pistons giving it a slightly larger bore and slightly higher compression. It has Toyota Land Cruiser valves, slightly larger than standard and the internal of the exhaust is more free flow than standard so the bike breathes easier.
The use of a BMW PCV hence does not phase me at all and if it might be considered a bit of over engineering I dont mind as it is doing the job very well. I have no, that is NO, leak from the timing case/magneto junction. In fact I was asked at the Centennial Rally in April in Australia where that leak was on my machine. It was pointed out to me that that leak has a part number in the catalogue! Oil leakage anywhere is negligible.
As for the question of the smoke, noise or any of the other problems: I received some good advice on pistons whilst I was restoring my machine. The problem seems to be that the original pistons (as I understand it) are round. When the pistons get warm they expand more where the metal is, ie around the gudgeon/wrist pin area. This non uniform expansion then causes the pistons to become out of round when hot. By the simple expedient of putting a threaded rod through the piston gudgeon holes and two nuts inside the piston can be spread slightly at the gudgeon area. By then turning the piston in a lathe to round it assumes a slight out of round when the nuts are removed but expands to round when running. I therefore get a bit of smoke at start-up but little in running. The engine also runs quietly. Several Douglas owners commented to me at the Centennial Rally at the quietness of the motor. Not once in four days of the Centennial Rally did I have to kick it more than once and have no plug oiling problem. One last point, I use Penrite Shelsey Medium oil which is formulated to replace SAE40 mono and I change the oil regularly. I also have a couple of BSAs and if you are going to drop oil it isn't as obvious if its clean!

   

[tommy]

Tommy,
Once again I am no expert and am, like you, a bit of a cynic. I do try to be a bit philosophical when it comes to British motorcycles though for the sake of my sanity.

   
I am no cynic on the workings of the machine or mods/improvements.
Don't give a stuff if you can see the mod externally and is not original, if it makes the thing go.
I ride them, sometimes long distance and sometimes very hard. If they don't take it, they break. Find a solution and move on.
Same with a racer or sprinter.

I am a cynic on the use of this site and the suitability of the mods for the average user to get to grips with the issues involved.

The use of a BMW breather is fine if it works for you.
I am not saying your solution is over engineered. BMW's are at times that way!

A big bore breather straight out the filler cap is within most peoples capabilities and coupled with a straight oil will get them through the running in period. Later the pipe can be removed as it is not needed.

Myself, I use Honda 750K pistons.
These are superb quality, low expansion alloy. No mods to the piston are needed. They do not cease, do not smoke and run as good as gold from day one.
No breather mods, no loss of power/compression, pulls like a train and you can not see it from the outside.
You can run without being frightened of the engine nipping up.
Far more important than if the engine gets a runny nose!

If you build the engine carefully with good gaskets and modern sealants, they will remain oil tight.
I have no breather on the bike (apart from Mr Douglas own) and I get NO, repeat NO oil loss or leaks around the cases. 
She starts first kick under normal conditions and I think nothing of setting off on a 500 mile trip on her, two up if required.
I will take her again this year to the Irish rally for a week covering tough terrain and about 150 miles per day.
Did it last year without missing a beat or changing a plug.
Two events will ruin a set of tyres.

Personally, I would be more concerned about the poor filter system for the oil and running open bell mouths on the carbs, sucking in muck.
Neither keep me awake at night or stop me running the bikes though.

If your mods work for you, great.

Enjoy!

Tom   

[eddie]

Regarding the subject of an oil leak from the joint between the magdyno and timing chest: Some crankcases suffer from the two faces that locate the magdyno not being exactly square to each other - that is the platform is not square to the back of the timing chest. However, the standard cork gasket will 'give' enough to compensate. It is relatively rare for the leak to actually occur at the joint - usually the oil escapes through the small drain hole in the magneto (on the underside, directly below the HT pickups) and spreads across the top of the crankcase as if it were leaking from the joint. The culprit is often an incorrectly fitted oilseal in the front of the magdyno. This seal should be fitted with the open side outwards as it is intended to stop oil getting into the magdyno, rather than preventing it's escape. Also with the seal fitted the wrong way round, the lip often only bears on the tapered lead of the pinion. This lead is only machined, not ground, so can quickly wear the oilseal until it becomes useless.

                                 Regards,
                                             Eddie.

[tommy]

A very good point.
Just to make sure, I use a thin smear of clear silicone both sides of the mag cork gasket.
Cork is porous and in my opinion will let oil past, especially if under pressure.
Oil will follow the path of least resistance.
The mounting bolts for the mag to timing chest will strip the threads for nothing, so you can not overtighten.
Cheap and fast to do, a bit bolt and braces maybe, but no problems arise after.
 
As before, if all gaskets and seals are in good condition and assembled carefully, there should be no problem.
 
Interestingly if you have a big bore breather fitted, you can feel the air displacement and rhythm of the engine if you place your hand near the end of the pipe when the engine is running.
If the pipe is clear plastic you will see oil residue and condensation along the bore.
It has to be doing something and it can only be good?

By the very fact Mr Douglas fitted a breather of sorts, would suggest that one was necessary?
How effective is another matter. (also linked to the general condition of the engine)
On the Plus the breather was timed. Another advance? but would also suggest that as performance went up, so did the breather requirements?
Hence BMW have a definite arrangement on their twins.
That said, they are large capacity with pistons like buckets running at higher speeds.
The battery and starter motor requirements are massive to get the beast to turn over.
That boxer /compressor motion is much more than a Douglas.

Mr Douglas was on to something.
Maybe it was not developed to the correct level. Maybe it was fine for the time.
Now with the benefit of modern alloys and materials we may see that the breather arrangement was OK. The fault lay other places and the breather a fast fix?
If Douglas had all the answers at the time, they may still be on the go now.

Hindsight is wonderfull.

From what I have found, the standard engine built well needs no mods to the breather, but I do accept that the better you can vent the crankcase pressures, the better it is for the engine. It can do no harm.

Using a BMW bit instead of the mesh in the timing cover is one thing, but the vent pipe is still a piddly little thing.
Shallow breathing.


Tom


     

                                 

[pjondeck]

I failed to note in my submission that besides the BMW PCV I am also still using the Douglas PCV arrangement in the timing cover as depicted in my earlier posting, not the simple mesh unit. I would suggest that any reduction in horsepower spent compressing air in the crankcase must translate to a gain in horsepower at the back wheel.
Next time I am out and about I'm thinking of removing the modification and see what happens. I'd love to be able to put it on a dyno and find out if and what the difference is with and without the PCV.
Peter

[tommy]

I understand.
From what I gather, you are using a valve in the breather to expell crank case gasses/pressure and the valve closes to stop air pulling back into the cases??
I look at the Douglas engine like this.

Imagine you have three lungs.
One for each pot and one for the crankcase.

The two either side for the pots have an intake/inhale (carb) and exhale via exhaust.
For every action there is an =/opposite reaction.

With the crank case, this is not happening.

You are allowing the crank cases to exhale, but not inhale. Why?
If it can not inhale, you create a vacuum and therefore drag and the engine trying to suck itself inward.

It could be argued that crank case pressure allows the pistons to move back up the bore aided by the pressure.
Then you could say that drag occurs by compressing the air/gasses on the inward stroke when only one pot is firing.
One piston is doing the work while the other is a dead leg. Imbalance now occurs.

Less drag and resistance to the pistons moving inwards should be a good thing? OK, with the free and easy motion you can over rev it.

Take the human body. You can exhale till you fall over, but sooner or later you need to inhale.
If you run up and down the stairs ten times, you get hot, perspire and start to breathe deeply.

Is an engine not the same the harder it works?

If the pressure in the crank case become too much, it will blow.
Hence hi performance diesel engines have explosion doors on the crank cases. Not talking the family saloon car!

There are some interesting questions that arise from this with air flow, pressure and breathing, but this will develop over time.

Meanwhile, let's keep it comming.

Tom

 

[pjondeck]

I understand.
From what I gather, you are using a valve in the breather to expell crank case gasses/pressure and the valve closes to stop air pulling back into the cases??
I look at the Douglas engine like this.

Imagine you have three lungs.
One for each pot and one for the crankcase..........

I do love this stuff, it gets the grey matter working and I might even learn something.
Interesting analogy with the lungs but I see the situation as having two components, the ABOVE the piston component and the BELOW the piston component (or outside and inside in the case of the flat twin of course).
The above the piston component is as you say, a progression of inhalations and exhalations. The below the piston situation seems to me to be independent of what is happening above. All it knows is that the pistons are coming in then going out. Whether it is on compression or exhaust stroke doesn't matter to the crank case, that's all being taken care of by the valves.
I take your point on Newton's fist law, action and reaction etc, to eliminate the losses caused by compression and depression one would have to bore a bloody great hole in the crank case and REALLY let it breathe. It would seem that there will inevitably be some loss of horses doing the compression/depression bit, so the objective of keeping the oil inside the engine whilst perhaps minimising the other effect is the driving factor.
I am aware that a lot of classic race guys like to have the big tube option coming out of the crank case so that the thing can blow and suck freely and thence minimise the power loss. This of course requires the carrying on board of a container to carry the oil emitted from the breather, not an attractive proposition in the restorers situation.
Have I overlooked something?
cheers
Peter
 

[tommy]

I do love this stuff, it gets the grey matter working and I might even learn something.

Same here, although I do admit I preffer riding them to working on them or talking about them!

The below the piston situation seems to me to be independent of what is happening above. All it knows is that the pistons are coming in then going out. Whether it is on compression or exhaust stroke doesn't matter to the crank case, that's all being taken care of by the valves.

Don't agree on this one!
What is happening below is a direct result as to what is happening above. Above the piston is what makes the pistons go up and down.
This creates an =/ opposite reaction.
As the pistons travel down moving/compressing air ( a result in what is happening above) the crankcase/lung wants to exhale, pushing air out of the cases.

As the pistons travel back up the crank case, this pulls air back in to the lung. Inhale 
You have to give freedom of air to allow to do both.

If the valves in the head do not open it will have the same result.
No air/fuel in through the inlet there will be no spent gasses out the exhaust and no power.

Air/fuel in and the exhaust fails to open there will be a big bang. (no where to go, bit like the cases)?

All three are compression chambers. Don't matter if it's fuel/air mix, exhaust gasses or just hot contaminated air in the crank cases.

Suck,squeeze, bang, blow you want in the bores.

What do you want in your crank cases?

Tom 

[pjondeck]

My contention is this, If you take the spark plugs out (or even if you dont) and run the machine along the road in gear, the scenario inside the crank case will be no different to what is happening under normal engine running conditions. Since we are only talking about the crank case I reckoned that we could therefore pretty much ignore what is happening above the pistons for the sake of the discussion. The objective as I see it is not to create a total vaccum in the crank case which I suspect would cause problems with ring blowby etc (but I am getting out of my depth here) but to reduce the pressure in the crankcase to below atmospheric pressure so that the pressure differential is biassed against oil being pushed out of the crank case.
cheers
Peter

[graeme]

You are absolutely right here Peter. The aim of the extra breather is solely to keep the oil inside the engine.
I have no experience with post war Douggies, but do know of their reputatiation for leaking out oil where they are not supposed to. I have added extra breathers to both a Norton Commando and my ride to work 500cc single cylinder Honda, as with both engines, despite being in good nick, they leaked oil out of the top end - the Honda particularly. These engines are notorious for this. I tried all sorts of things to stop oil leaks from the rocker spindles, with no success. An extra breather from the rocker box cured the problem instantly.
I should also make mention of the fact that most car breathers have a one way valve, designed specifically to keep a low pressure inside the crankcase.

[trevorp]

OK my point of view is like this first take into account compression ratio as what goes on above the piston is different to what happens below the piston in terms of pressure above the piston may be 100 psi and below in the crankcase it may only rise by 5 psi not including blow by past rings which as they get more worn the higher your crank pressure will be
know if we stay below the pistons we are getting 350cc of air displaced in the crankcase every downward stroke so if we take the compressor theory it is not a good compressor in standard trim because the inlet and exhaust r the same thing the tube in front of the timing case
It was my impression that the crankcase pressure stayed slightly positive but in cavitation as surely 350 cc of displacement don't go in and out of the small tube at 5000rpm maybe thats why the made it that size
Pete what i believe you are doing is making the compressor below the pistons more efficient by adding a much bigger hose and u are now adding valves to the compressor making it more efficient
Also i believe when Eddie was talking about pressure waves which is what happens in cavitation the reason Douglas put the tube so far away and a rather tortuous path is possibly to try and reduce the effect of these and the timed valve arrangement was to try and stop and regulate these pressure waves
in 2 stroke theory in exhausts u can reflect pressure waves back to the cylinder to hold back slightly the incoming charge so there may even be a pressure wave timing issue which is above my knowledge but now u have interupted that timing i have no idea what that will cause
Think of an electric compressor that has a 1 foot piece of 1/4 tube connected to the inlet and and used as an exhaust it wouldn't be very efficient now add as u have a 3/4 inch outlet with a valve u have made a much more efficient compressor
Now if u block the 1/4 tube u have created a vacuum pump
if u create a vacuum in the crankcase it will suck air at its weakest point and that maybe the magneto seal which wont leak at first but if it does get by there dirt and dust will destroy it and start an oil leak
my best suggestion is someone do a compression test on crankcase to see what it is through the rev range and if crankcase breathing is the issue it may be best on your Pete to lose the valve and just use the tube u have without the reed valve
also u need to know the pressure the Pcv valve seat lifts as the spring will be set to a certain pressure setting if u use one
but the more efficient the compressor below the pistons the better the crankcase ventilation but costs horse power and possible overheating
Positive crankcase ventilation was created to take unburnt hydrocarbons and re-burn them through the inlet manifold and stop the venting of hydrocarbons to atmosphere for anti pollution purposes and stop oil contamination but they have a filter on the inlet side for air into engine to stop dirt and dust entering not like your Pete which has a valve to stop the inlet so it will become a negative crankcase ventilator valve
OK that should get the grey matter working and my face slapped at the same time

[tommy]

know if we stay below the pistons we are getting 350cc of air displaced in the crankcase every downward stroke so if we take the compressor theory it is not a good compressor in standard trim because the inlet and exhaust r the same thing the tube in front of the timing case
Pete what i believe you are doing is making the compressor below the pistons more efficient by adding a much bigger hose and u are now adding valves to the compressor making it more efficient

How is Pete making a much bigger hose?
The PCV valve maybe letting more air past the valve than the mesh because the opening is larger? (is it)?
You are still restricting after the valve with the vent tube being the same size as standard, running down the front of the timing chest.
A much bigger hose is a much bigger hose. I don't see a much bigger hose!
A bigger diameter than is already there and preferably the same diameter as the valve, if not bigger to exhaust the gas/air away. Yes.
But you are still using the original.

To me, when the pistons travel inward, you are compressing the air. It is this action that will fire oil under pressure to anyplace it can find.
Air pressure will give oil a hydraulic affect.
As the pistons move outward they are relieving pressure in the cases, providing they can take air from somewhere.
The oil will stay put at this point. With relief to pressure it will drop to the bottom of the cases apart from the crank splashing oil around and the oil pump doing its natural job.
By stopping incoming air, you create a vacuum causing the oil to stay in suspense. Still don't see how that will force oil out, as if anything it will pull it back into the vacuum from outside.
Bit like a big bubble of snot on a kids nose. As they blow out the bubble expands and as they breathe in, it retracts.

OK that should get the grey matter working and my face slapped at the same time

Grey matter working and no face slapping!

Gradually we are getting there.
 
[/quote]

[trevorp]

yep i think Pete and he will know has about half inch hose off the pcv valve and he doesn't have the mesh on the timing cover he has the one way valve, so theoretically he has no inlet, only 2 exhausts valves for crank pressure
This will create a small vacuum in crankcase but it will eventually find somewhere to suck air from i would imagine, and at high revs a pulsing action in the form of a pressure wave has been known to suck through oil seals as most are designed to keep pressure in not be subjected to a vacuum
i think the Douglas engine was, and still are to have a small amount of positive pressure in the crankcase this pressure lets say for theory is 8 psi then that is what the pcv valve spring should be set at, so it relieves at that pressure
The main thing i think to really solve this problem is i don't have my Mk1 motor back together yet but when i do i will record some data from it vented unvented and normal crank pressure and get a baseline to work off
i know this is a real longshot but who was the engineer that actually designed the engines and is there any real technical information on the mark series engines did any of the engineering stuff survive
 

[pjondeck]

Yes, I am running both the BMW PCV with a 12mm ID tube and the Douglas PCV in the timing cover. I am not aware that there is a deliberate "inlet" to the crank case, otherwise why would Mr Douglas have put valves in the timing case unit?
The Douglas unit I stripped, cleaned and reassembled and feel that it is working as well as it ever did. It relies on 3 small light discs that with air inhalation are drawn to close their relative holes in the unit.(see earlier photographs this topic) There is no spring pressure. I think that the efficiency of this unit is iffy at best and such that there is never going to be a strong vaccum in the crank case. The thing is not modern engineering with poofteenth of a thou tolerances. The suggestion that gaskets are designed to restrict flow of air, oil or whatever in one direction only I find difficult. A gasket is a gasket is a gasket to my mind. The whole question of crank case ventilation comes down to one priority, keep the oil IN. If by doing so there allows some foreign material to invade the crank case by creating a strong vaccum therein I think that it would be so minimal that the problem would be easily overcome by the conscientious changing of the oil. Anything that does enter the crank case I would think to be of less concern than that entering the upper cylinder via the carburettors. I think that's about all I have to offer gentlemen, it's been fun.   

[trevorp]

Not talking about gaskets Pete talking about seals as in the rubber type or felt with lip or variations, gaskets r gaskets doesn't matter about them they can take load in any direction
ill do some tests we will restart it after that if i find anything u should know about, true change the oil and keep riding it and post back after the bike has done a few thousand kms thats probably the best test
and I'm done too on the subject

[tck]

[The essence of the thing is commercially available at modest cost]
Brian

Well I read it all and the comments but could somebody reiterate the 'commercial' bit
I cannot see a reference
I glean its a BMW part : do we have the part number?

[Brian]

Hi tck,

"Breather Valve" component is BMW Part No. 11400 Engine Breather Reed Valve
If you are in the UK ,you can buy from Moto Bins Ltd   sales@motobins.co.uk £7.50 + carraige+ VAT

Other parts are turned Al Alloy Bar etc.Full details if needed

All the best (it works a treat)

Brian T

[pjondeck]

I purchased the BMW PCV from Munich Motorcycles in Western Australia.
Delivery was prompt and cost Aus$29.50 incl P&P. See attached site.

http://www.munichmotorcycles.com.au/

[tommy]

After being invited to get stuck into this subject, I did so and a little tongue in cheek took the subject around a route to get people thinking whilst using terms for the layman with lungs etc.

This has brought about many different points, that when put together give the answer.

Mr Douglas did know what he was doing.
In answer to a recent question, I understand Freddie Dixon was brought in to transform the performance of the MK series changing the draught of the carbs, pistons and cylinder head design etc after the MK1.

A MK with an engine in good condition need no further breathing device other than standard.

The plus series has a timed breather which is the best way and needs no further alteration.

If you have no timed breather and the issue keeps you awake at night, a flap valve such as BMW is the next best thing.
If it is good enough for BMW with much bigger CC then it should be good enough for 350cc. Even if you are messing a bit by trial and error.

If you can not do the BMW mod, the next best thing is to fit a big bore breather as it is simple and cheap. You will loose power though and as Eddie has said, you are masking a problem elsewhere.

You need the engine to breathe as best as possible as the pistons travel down, expelling air/gasses and pressure.
It is best when they travel back up to have a slight negative pressure/vacuum in the cases as this stops you loosing power shifting another 350cc of air.
The neg pressure sucks oil back down the bores keeping it inside the cases.
This also stops dirt etc being sucked back in.

A timed breather is more accurate as it works in harmony with the engine rather than a delayed reaction to air pressure resulting from the mechanical action.

Triumph even had it on their cub engines and everyone presumes they did not know what they were doing back then.
They did!

Modern motorcycles and designs are not very different now to then.
Materials and more accurate machining (CNC etc) fuels, oils, gasket materials etc etc etc have all contributed to making them more reliable, oil tight and faster.

On the whole, a motorcycle racing round the Isle of Mann still has two wheels, chain drive, an engine and gearbox, clutch, fuel tank, frame and seat. Still on telescopic forks even now, although placed upside down with valves to alter the ride.

Oh! and disc brakes, but Freddie Dixon had disc brakes in the twenties.
Now we have four pot, six pot calipers etc and Carbon discs.
The principles are the same, just modern materials.
Tyres is another great example.

Most of the oil loss sustained by British bikes came from poor quality porous castings, not from breather technology.

That, happily is me also done on the subject.

Phew!

Happy restoring and riding.

Tom
 



 

[pjondeck]

Thought I'd done with this but owe it to Brian to report my experience with his modification.
As reported previously I had done several hundred miles with Brian's BMW PCV mod during the Aussie Centenary Rally. The motor was dry and negligible oil from the breather tube at the bottom of the timing cover. There was about 50ml in the catch bottle in the "battery box", handy for the chain saw!
Yesterday I took the bike up to Bathurst to meet Jim Scaysbrook, the editor of Australasian Old Bike, for a photo shoot for the front cover of the next mag. The next mag will be released in late September and will have a five or six page feature on the marque. There will be a number of photos and I believe stories from the Centerary Rally. I digress; To maintain the originality of the Mk3S for the photo shoot I removed the PCV mod and reinstalled the brass cap.
After about 30 miles I had oil leaking from the magneto/timing cover seal and a small quantity at the timing cover breather.
I have heard all the arguments about what Mr. Douglas would have done etc., but I was pretty careful about the assembly of the machine in the first place and think that my sealing of the motor was pretty right. What I am saying is I have already refitted the mod and it will be staying there. It is easily removed if the bike is to be displayed and originalty is a concern but operationally I'm FOR IT. Thanks Brian.
regards all
Peter   

[graeme]

Hugh Whitmore told me that the oil leak at the magneto/timing cover seal has a Douglas part number!