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DT Douglas engine rebuild

Started by halihat, 06 Nov 2004 at 12:10

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I am just starting on a basket case DT engine rebuild. The engine is to go into a 1946 "500" racing car called Buzzie.

I have noted the earlier threads about cranks and con rods. Most of the parts seem to be there, but already I can see I need pistons and crankshaft bearings. Any suggestions or ideas?

BTW I am already an associate member of the LDMCC


More information about the Buzzie project here



Crankshaft bearings

Drive side- Originally designed for a Ø62xØ30x21mm double row radial ball unit that even during the late 1920s became obsolete.  Subsequently replaced by the factory with a pair of Ø62xØ30x10mm single row ball units separated by a 1mm shim.  Those are not available either!  

Timing side-  Ø62xØ30x13mm, also not available!  

So what to do?  

For the timing side purchase a Ø62xØ30x16 unshielded/unsealed maximum capacity single row ball unit like a Fafnir 206W.  Then take it to an engineering shop, have them pack the cage with oil soluble clay, and then surface grind the bearing to 13mm thick.  It will just end up touching the cages.  Do not omit to have the internal and external radius put back on the races, this can be done with a CBN tipped tool or by grinding.  Thoroughly de-magnetize the bearing; then wash the clay away with solvent.  

For the drive side there are two schools of thought.  One is to buy something like a MRC 5206M-H501, Ø62xØ32x23.8mm double row radial ball unit and grind it down to 21mm wide.  The second is to buy something like a FAG 22206E.C3, which is a Ø62xØ30x20mm double row self-aligning roller unit, and stick a 1mm shim between it and the crankcase.  This is the route I prefer, as the roller unit has a much higher load capacity than the ball unit.  Of course being self-aligning, it offers no rigidity to the crankshaft.  But, considering how thin the drive side crankcase wall of the DT/SW engines are, letting the crankshaft spring a little bit is better than cracking the case.  And they do fatigue crack around the bearing housing on the drive side, so check yours.  The Douglas OHV crankshafts are incredibly springy, which you will find out if you ever do have to straighten one!  

Douglas did use a double row self-aligning ball unit on their side valve engines; the self-aligning roller bearings not being available then.  However in this case it was probably to allow for inaccuracy in alignment of the main bearing bores in the two crankcase halves.  The narrow single row radial ball unit on the timing side (often only 10mm wide) had a slight amount of inherent 'give'.  

For a parts list, there is only one extant and it is unfortunately not an illustrated one.  Also typical Douglas spares list, a few bits got left out by the factory!  See John Wither's advert "Douglas Motorcycle Repro Books" under the 'Douglas Products and Services' section of this forum (catalog item VIN207, under OHV subsection.)  

For pistons I turned down some old used Hepolex items, that probably were for a 63mm bore engine.  The standard DT/DT 500cc bore is 62.25mm (82mm stroke) and often the first re-bore takes it out to 62.75mm.  I used BSA model C15 (63mm bore) rings, re-gapped.  You might be able to use a C15 piston, though I have never had the opportunity to check the pertinent dimensions.  The main difficulty in finding a match lies in the short crown height of only 7/8" to the top ring land (excluding dome) and the valve pockets in line with the wristpin rather than perpendicular like most engines.  Also depends on what type of fuel you plan to run.  The original pistons were quite high domed, but then they were running 50/50 petrol-benzol.  They have been setup recently anywhere from 6.5:1 to 8:1 for petrol and up to 12:1 methanol.  

By the way, I checked out Buzzie's website, what a great project.  I have only ever seen one other 500cc Douglas rear engine car, and thought it a neat concept.  Best of luck with it.  



Thanks Dave for putting up my Buzzie website url. I am expecting to finish the next stage of the restoration soon, so will be posting an update on the restoration news area when I get that done.

Thanks also to Doug for the extensive information! A lot to get my teeth into! I need to go shopping, so that when I have got the chassis moved on, I am ready to get the engine project moving. I have half an eye on the proposed "500" race at Goodwood next autumn!

I have done some more work on the contents of my basket, and discover I have a few more issues to resolve. Study of a few "New Con Rods" alerted me to the fact that the camshaft also runs in race bearings (I am new to engines of this vintage, and had assumed it ran in bush bearings!), and of course I have no information on them either! I also notice that the cam retaining plate is missing, so I wonder if it also acts as part of the bearing housing.

I am also a bit mystified as to how the cylinders are sealed. It looks like the heads are sealed with a wiils ring, as there are grooves machined in the top of the barrels, but I notice the parts list shows a head gasket. Is this needed as well as the ring? Also in my basket are two spacer plates (approx 70thou thick) that match the barrels. I can find no mention of these in the parts list, but do show another gasket to seal the barrels to the crankcase.

I notice that my engine has been bored to 62.75, but luckily the bores are unworn now. Also the concensus of opinion here with the bike racers I have met is that I should run the engine on alchohol. Apparently it runs cooler, and contemporary reports on Buzzie indicate it had a probem with overheating of the rear cylinder when installed in the car. The carb jetting would indicate my mine was set up for alcohol. I have several camshafts, and plan to start with  the standard DT 10/50/63/20 version.

Whilst my crank is in reasonable condition, it is heavily marked around one of the lands on which the race bearings run, so I suspect a bearing has been rotating on it. I may have to resort to metal spraying if I cant get a snug fit on my new bearing.

Finally (for now!) I havent got nuts for the cam or the crank. Does anyone know what the threads are?


Douglas Mclay



Ah well, cam bearings.  There you are in luck as that is still a size manufactured.  The camshaft uses a single row radial ball unit of Ø5/8xØ1-3/8x9/32" at each end; or what use to be known as an inch size R10.  The question you should ask is where they are made.  For some odd reason, the majority of imperial size bearing are made in Asia.  Even bearings from the house hold names in the ball bearing world are made in China, re-packaged and significantly marked-up in price.  Check to see what is etched on the bearing itself as a country of origin, and not what is on the packaging.  If you are not worried about where they are made indicating the quality of manufacture, you can buy the same bearing for much less under the trade name "General".  They make them for all the well known Japanese, German, and American firms.  

Two alternatives are a "Bearings Limited" E15, which seem to be made in Slovakia and perhaps the best choice, "RPH" number KLNJ5/8Y, which are made in Switzerland according to the last batch I got.  Note, this only applies to imperial sizes, which are becoming a low-volume product.  Many of the big firms like to keep a much tighter control on their metric bearings, and do not outsource them.  Also the big names do have their own plants spread around the globe, so you never really know where they are going to come from next.  

The cam bearing on the timing side half of the case is retained by a simple steel cover plate, affixed by two 1/4-20 bolts either side.  This closes off the bore, which on the timing side is a through hole.  I have seen them flat on the side facing the ball race, and recessed.  The latter I suppose when they messed up boring the cam bearing housing depth in the drive side half, and there was insufficient end float for the bearings as a result!    

These bolts have a rather shallow head to fit behind the cam gear.  They are cross drilled for safety wire, which is rather ridiculous, as one can never wire up tight between the two bolts as the cam is right between them!   Modify the plate design to take tab washers or use a low strength (non-permanent) thread lock fluid.  

The head gasket is a solid copper wire of diamond cross section which fits in the grooves you noted.  Some folks just use round cross section wire these days.  It develops flats and in time conforms to the groove.  I have been using square section wire used for winding large electric motor armatures.  This I lay in the groove flat, and like the round wire it gets crushed to conform to the groove.  I roll the rings up slightly smaller than required, and then weld the joint with copper wire and a tungsten inert gas welding torch.  After dressing the weld flush (easier in square wire than round), I press it over a mandrel that expands it out to the correct diameter, and gets it nice and circular again.  It probably would not bee too much trouble to roll the wire on edge and so make the diamond cross section as per original; but a slightly larger square wire would be required.  But I never bothered.  The important thing when sizing the wire is that the flat surfaces of the head and the cylinder must not touch, there should be a slight gap so that you are always clamped up on the gasket, or joint ring to call it what it really is.  If you are crushing the wire, you also have to allow extra as it will settle over time and the gap will close up; maintain at least 0.010" and change the joint ring out if it becomes less.  

Two other options are first, just to use flat copper sheet gasket and ignore the joint ring completely.  Earlier Douglas o.h.v. engines did not have the joint ring, nor a spigot.  I do not know if they used a solid copper gasket or a sandwich composition.  I have heard of folks using just solid copper.  You may need to flatten the head surface if you intend to go this route as the head lugs may have pulled down over the years ever so slightly.  But the cylinder should be fine as it is in compression and there is no stress through the planar portion of the joint surface.  The second is to turn joint rings from solid, tube if you can get it (expensive), or plate (expensive as well!)  

All the used rings I have seen so far are all solid, but I can not say if they are original ex-factory, or if Douglas possibly made diamond joint rings with an asbestos core.  I have had a set of the welded rings in a friend's touring SW for some fifteen years, and another set has been vintage racing in the UK on road circuits.  If you could get a Wills ring big enough in section and clamped the joint faces up tight, you would have the best of all worlds.  

Between the barrel and the crankcase should just be a paper gasket.  Douglas seemed to favor 0.006" thick material.  The shims you have are something someone made to play with the compression ratio.  Or perhaps someone has skimmed a bit off the base flange, a very common occurrence over the years, and needed to restore height.  For the 62.25x82mm DT/SW engines the distance from the base flange to the head surface is 4.825".  There are several types of barrel, but the two mostly likely encountered are different in the base flange.  The early type is thinner, and has four raised bosses where the cylinder through-studs pass.  These are prone to crack if shortened too much.  I was about to set a record time at the Wroughton airfield vintage sprint (well, perhaps not  :D ) on a borrowed DT Douglas and was let down by this very fault.  You will be surprised how much oil can escape from such a small crack.  A later style has a base flange a uniform 5/16" thick, and so is stronger and has more scope for re-cutting.  

Douglases reputably had overheating trouble when they re-entered the TT in the thirties and had to run on straight petrol.  It is said the course fining was designed in the twenties with the 50/50 fuel then in use in mind.  Be that as it may, they do not overheat in touring use with the same fining and most folks using a Douglas in anger today are sprinting in 1/4 mile races so there is no cumulative heat build up.  If you are planning to use the car for demonstration laps or hill-climbing, you probably will have no overheating problems, depending of course on how shrouded the engine is when installed in the car.  But it will run cooler on alcohol or methanol.  The late Bill Dent swore by methanol in his famous DT Douglas sprinter.  He also said it was gentler on the engine; a softer combustion in the main burn sequence.  

The thread on the end of the camshaft is 3/8-20, or it is on the 8-spline drive types.  They also made an earlier taper drive and a later small spline drive that may have used a different thread.  The crankshaft thread is 7/8-20.  Douglas, when not using their own threads (and they had some wacko threads) were partial to the Cycle Engineer Institute thread series, which at that time included two separate systems based on twenty and twenty-six threads per inch.  The later of which is all you can get taps and dies for today.  Still Douglas applied it to diameters I do not think the CEI system intended!  Earlier o.h.v. cranks had a smaller taper and thread.  Also over time many have had the threads damaged and re-chased, and are not longer exactly Ø7/8".  Even the ones that are tend to measure closer to Ø0.865" due to the thread crest being worn down.  



Thanks again for the additional info. Looks like my local bearing house is going to be busy!

V interested in the info about running hot etc. What you say rings true with the contempoary reports in the magazines of 1946/7! Apparently Buzzie ran quite well in sprints and hillclimbs, and even got a couple of class wins. However in races it was a different story. I cant find any record of Buzzie actually finishing a race, usually failing due to overheating or misfiring, which could also be due to overheating. Of course we have no idea what condition the DT engine was in, but Buzzie was built during the Bristish post war austerity period, and judging by the quality of the work on the chassis, and the lack of funds the whole project laboured under it is unlikely it was returned to the Douglas racing shop for a rebuild! In any event it may be that hillclimbs may be the cars limit when its finished. Also I have no idea whether it ran on petrol or alchohol. I get the impression it was dope, but the 500 club magazine refers to the effects of petrol rationioning on racing, but that may just have been used to transsport the cars to and from the track.

I have looked into the BSA pistons. However I dont quite understand the phrase "the short crown height of only 7/8" to the top ring land (excluding dome)". Is this the distance from the centre of the gudgeon pin to the top of the first land? If so any idea how much dome can be accomodated? Perhaps another way of evaluating the pistons would be to know the distance fron the centre of the gudgeon pin at TDC to the top of the bore, so an estimate of compression ratio could be made?


Douglas  MCLay



The 7/8" distance I mentioned is from the center of the gudgeon pin to the top of the first land.  I should have just said that in the first place and left the word 'crown' out of it.  Will try to sort you out some info re-crown heights proper, but these days it will be take what ever you can get!  



Hi All,
         I have just noticed a misprint/error in an earlier posting on this subject. I think the piston rings would have been from a BSA C11 or C10 at 63mm bore - the C15 had a 67mm bore. Recently, when I rebuilt my 500 DT motor, I used rings from a pre-unit 500 Triumph - again 63mm bore - and much more easily obtained here in the UK! C15 rings at +.040" are OK in standard size 600 barrels or 500 unit Triumph rings (69mm) can be gapped down and used in re-bored 600 barrels.