Author Topic: Douglas Aero 1937  (Read 42487 times)

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Offline Eric S

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Re: Douglas Aero 1937
« Reply #250 on: 09 Sep 2019 at 17:32 »
I finally went ahead and prepared the parts to put them together.
I am anxious as I never went so far in disassembling an engine and would like to avoid making mistakes in re-assembling.
Plan is
1- install the bearings
2- Close the 2 Halves on the crankshaft.
3- Install the pistons and cylinders
4- Install the timing side
5- install the oil pump side and generator.

And what do you think of my pistons? I have 2 scratches on the side.

Sorry pictures are a bit large :roll:

Offline Aero

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Re: Douglas Aero 1937
« Reply #251 on: 09 Sep 2019 at 18:59 »
Hi Eric,
the marking on the pistons would suggest to me that there is insufficient piston to bore clearance.
I have had similar issues with my engine & had to hone the bores twice.
It seems these engine need a lot more clearance than most machine shops realise.
At the top of the piston just below where the rings sit you need around 6 to 7 thou clearance, and at the bottom of the skirt you need around 5 thou clearance, piston to cylinder, measured at right angles to the piston pin.

It would also be a good idea to have a trial assembly of the crankshaft, bearings & crankcases, to see if you need to shim between the crank & the bearings to get the crankshaft axial clearance down to a reasonable level.
Mine needed around a millimetre each side, although I have also installed a thin 0.15mm paper gasket between the crankcase halves.
Originally there would have been a 1mm thick oil thrower washer installed on each side of the crankshaft, which would account for the need for shims around the same thickness.

Offline Doug

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Re: Douglas Aero 1937
« Reply #252 on: 09 Sep 2019 at 23:34 »
Eric,

I am personally not a fan of plastic cages for bearings used in engines. Particularly ones where you often have to heat the case up to drop the bearing in and out. Also, the one on the timing side looks to be very light duty, given the few number of bearing balls.

As mentioned already, you will need to assemble the crankshaft in the cases at least once to test the end float. There are ways to pre-measure this, but most folk do not have the measuring equipment and fixturing to do it. I forget now if the Aero 600 was still this way, but earlier engines were designed for a 21mm wide bearing that even then was becoming obsolete so they incorporated a 1mm shim with a 20mm wide bearing. Even if that was addressed, they still typically bored the flywheel side deep so as to allow for a few shims to get the end-float just right. The shims go between the flywheel side crankcase and the ball bearing.

The pistons look like they have been clawed by a bear! That is more than what would be described as a few scratches... Ease down the high spots and reuse with new rings. It is not ideal, but probably you do not have much choice as pistons are getting hard to come by. Is that recent or old history? If recent, then there is still a problem that will need to be addressed as to why the pistons seized.

-Doug

Offline Eric S

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Re: Douglas Aero 1937
« Reply #253 on: 10 Sep 2019 at 07:04 »
Thank you Doug and Aero
You point out the weak design of the bearing. Maybe I shall use the old one that was not really bad.

As for the pistons, they have no history. I just removed them to work on the crankshaft and have the flywheel cone rebuilt. I just took them off and put them aside.
I didn't had any trouble with the engine as far as I can tell. Only problem was the clutch.
There is no matching seizing marks in the cylinders as far as I can see. Even though the cylinders are not nice. I will send pictures of them later.

Do you have links to get rings?

Offline Eric S

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Re: Douglas Aero 1937
« Reply #254 on: 10 Sep 2019 at 16:59 »
Here are the pictures of the cylinders. 2 of the same cylinder as it came off the engine. One slightly honed.

Offline Aero

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Re: Douglas Aero 1937
« Reply #255 on: 10 Sep 2019 at 20:06 »
They look like they need a bit more honing. As mentioned previously check the piston clearances to see how little or much you can hone. For the last 10 seconds of honing, with the drill on slow speed, move the hone rapidly up & down the length of the bore, this will give you a nice "cross-hatch" finish that will help the rings bed in quickly. There is a place near me (Shropshire) called FW Thornton who will supply piston rings worldwide.

Offline Eric S

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Re: Douglas Aero 1937
« Reply #256 on: 11 Sep 2019 at 07:46 »
Thank you for your help.
I called the mechanic that made the cone and I will visit him on Monday so he can have a professional look and measures of cylinders and pistons. He will check for roundness or lack of the cylinders.
I will have him check this tiny bearing too and the rest of the engine.

In the meantime I would appreciate to get the factory dimensions of the pistons so I can check if I have originals or replacements as well as the bore dimensions.

Offline Eric S

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Re: Douglas Aero 1937
« Reply #257 on: 16 Sep 2019 at 09:54 »
Anybody could tell me if those pistons are originals?
Anydimesions available?

Offline Aero

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Re: Douglas Aero 1937
« Reply #258 on: 16 Sep 2019 at 18:31 »
They look like Hepolite replacements, the same as mine.
Standard bore is 74mm, with oversizes at 20, 40, & 60 thousandths of an inch, so roughly 74.5mm, 75mm, & 75.5mm.
If you clean the piston tops the oversize should be marked "20 thou" etc on the crowns, together with Hepolite or Heplex. They are unavailable new these days.
« Last Edit: 16 Sep 2019 at 18:40 by Aero »

Offline Eric S

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Re: Douglas Aero 1937
« Reply #259 on: 19 Sep 2019 at 07:12 »
Do we know if Hepolite replacements are the same dimensions everywhere as the originals. We may have to chase for replacements so need to know what we are looking after. If we found replacements that are different than the Hepolite but same as originals it would be too bad to miss them.

Offline Aero

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Re: Douglas Aero 1937
« Reply #260 on: 19 Sep 2019 at 11:40 »
Original (I assume) on the left, Hepolite on the right

Offline Aero

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Re: Douglas Aero 1937
« Reply #261 on: 19 Sep 2019 at 11:43 »
Underside

Offline Eric S

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Re: Douglas Aero 1937
« Reply #262 on: 30 Sep 2019 at 09:14 »
I talked to the engine builder this morning.
Cylinders are measured at 74,75mm. Pistons 74,60mm
Cylinders' walls are porous and he's advising re-boring.
However how can I get new pistons? What are the options? He advised against custom made pistons as quality is not really good on his experience...

Offline eddie

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Re: Douglas Aero 1937
« Reply #263 on: 01 Oct 2019 at 07:29 »
Eric,
        If the bores are not marked, don't get them rebored - rebuild the engine with the old pistons, but fit new rings. With .15mm clearance, they should be fine! Ignore the comment about reboring because the bores are 'porous' - taking another .25mm cut wont make them any better - in fact, 'porous' bores will retain more oil and prevent further 'nipping up'. Also, don't get too paranoid about the quality of repro pistons - the originals wouldn't have been any better.
    When you rebuild your engine, build it on any cheap motor oil until it is 'run in' - then go over to a better quality oil. Modern oils are so effective that the running in process doesn't get completed before any high spots become glazed. ( I used to work on marine diesels, and one of our craft had to have all the liners and pistons replaced under guarantee because too good an oil had been used during the running in period, and the bores had become glazed!)

  Regards,
                  Eddie.

Offline Eric S

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Re: Douglas Aero 1937
« Reply #264 on: 01 Oct 2019 at 13:59 »
Great thank you Eddie for bringing some good news.
I assume it still have to be honed though??
Just in case so I/we know, what would be the options should one need pistons?

Offline Aero

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Re: Douglas Aero 1937
« Reply #265 on: 02 Oct 2019 at 10:51 »
As Eddie has already said, 0.15mm is a good clearance so best to just lightly hone any marks out of your cylinders & re-use the old pistons.
I have a couple of good used pistons measuring 74.7mm if ever you get stuck in the future, but I'm holding on to them for the moment in case I find any spare barrels.

Offline Eric S

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Re: Douglas Aero 1937
« Reply #266 on: 21 Oct 2019 at 18:04 »
I got the engine parts back.
They surfaced the heads, the top of the cylinders and honed the cylinders somewhat.
As said earlier, he was concerned about the surface quality of the cylinders being porous but I asked him just to hone the cylinders without re-boring them so he did the best he could.
They supplied new rings but did not cut them. He told me to adjust with a 0,4mm/.016" gap. Is that OK?

Offline Aero

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Re: Douglas Aero 1937
« Reply #267 on: 22 Oct 2019 at 17:27 »
It looks like your engine builder has used long honing stones.
In this instance I think a flex (ball) hone would do a better job.
Somewhere between 12 & 16 thou is ok for the ring gap.
If you gap them now at 12 thou by the time they've bedded in they will probably be more like 16.

Offline Eric S

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Re: Douglas Aero 1937
« Reply #268 on: 04 Nov 2019 at 09:04 »
Looking at reassembling the engine now and here is a first question.
The bearing on the crankshaft, clutch side is a tight fit and slides in tightly. Surface on the shaft is rough. Removing the burrs may make the bearing loose on the shaft.
How would you do it. Install it with some kind of compound, dry, file off the burrs?

Also I have an oil sling on the same side on top of the bearing. How can I measure and determine if I need any more spacers/washers. How close does it need to be?

Offline Eric S

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Re: Douglas Aero 1937
« Reply #269 on: 11 Nov 2019 at 21:07 »
No Idea?

Offline Doug

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Re: Douglas Aero 1937
« Reply #270 on: 13 Nov 2019 at 01:20 »
Eric,

The state of this engine continues to dismay!

I would stone off the burrs. It is a similar situation to the practice of center punch marking shafts. Besides being a deplorable practice, over time the high spots get compressed, resuming the original slack fit. Better to know now if the fit is slack and not providing the proper support and remediate it.

I do not know if this is worth the trouble for a single engine, but I made a simple gauge to measure the end float of the crankshaft for the DT engine since I have done a several and have a few more to do. This is a cylinder of aluminum turned to just a little under the ball bearing outer diameter so that it is a slack fit . Both ends are faced square with a chamfer equal to the bearing radius. The length is not critical other than it needs to be a little less than the distance between the bearing bores. To use, the measuring cylinder is inserted into where the bearings would go and the crankcase bolted up tight with any gaskets that you will be using. The measuring cylinder is push all the way to one side until it bottoms and a dial indicator is arranged to contact the end face and the indicator zeroed. The measuring cylinder is then pushed to the other side until it abuts and the indicator reading of the distance traveled observed. The distance inside the crankcase is the indicator reading plus the length of the cylinder. Then I assemble the bearings and slingers onto the crankshaft and take a measurement across the outside. I then can figure out how much shim is required to achieve the desired end float. That enables me to set the end float in one try. It requires several things not everyone may have available; a bar of metal, an lathe, and measuring equipment.

The traditional way is to assemble the crankshaft into the crankcase and using a mallet, thump the crankshaft to one side and measure the distance from some reference point on the crankshaft relative to a reference point on the crankcase. Then using the mallet to shift the crankshaft to the opposite side the measurement is repeated. That tells you the end float of the crankshaft. You then add or remove shims from behind the bearing races (usually on the flywheel side if both mains are ball bearings) to correct end float. The nuisance is the bearings are a shrink fit in the cases, so you have to go through several sessions of heating the crankcase to pop the bearing in and out as to experiment with shims.

-Doug

Offline Eric S

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Re: Douglas Aero 1937
« Reply #271 on: 14 Nov 2019 at 15:22 »
Doug

thank you for this information
The thing is the shaft has no really burrs or high spots. It's just not smooth. Bearing slides in and out without feeling any rough surface. It just pops in and out. It's a bit tight and I have to play it around so it can slide in. Making the shaft smooth would have some play...

Not sure to understand though. The bearings are inserted in the case and then shims are between the bearings and the crankshaft. It was this way when I took it apart. But why do you say "you have to go through several sessions of heating the crankcase to pop the bearing in and out as to experiment with shims".
Seems to be the opposite than "behind the bearing races (usually on the flywheel side if both mains are ball bearings)"

Then wouldn't it be easier to fit the bearings first and using the aluminum rod sliding bar method measure the required length of crankshaft+shims?
Those are going to be tiny shims but do we want one on each side to center the crankshaft or is it not important?
« Last Edit: 14 Nov 2019 at 15:27 by Eric S »

Offline Doug

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Re: Douglas Aero 1937
« Reply #272 on: 14 Nov 2019 at 16:15 »
Eric,

The factory installed the shims between the crankcase and the bearing. There are slingers (in some designs) between the bearing and the crankshaft, but these are not intended to be shims. Presumably Kingswood (and other factories) did this to protect the thin and delicate shims. If they were installed on the crankshaft they would be more prone to damage during assembly. The crankshaft journal has a fillet between it and the throw to avoid a stress riser. If the shim registered on the crankshaft, the hole through would have to be bigger than the journal so that it would clear that fillet. During assembly you would have to be careful the shim rode up that fillet and registered on the flat face, else it might get pinched and not allow the bearing to seat fully. Also they might have figure the crankshaft shuffling back and forth the amount of the running clearance would unduly hammer or at minimum compress pinch the thin shims as the crankshaft flexes, so behind the bearing they are static as well as better protected. It seems to be normal practice, not just with Douglas.

I actually do not remember a slinger(s) on my 1936 Aero, but it has been a while since the engine was assembled and I did not take a photo of the assembled crankshaft that I can refer to. Most designs have a shallow spigot for the slinger to register on, just for the purpose to keep it off the journal radius. I do have a picture of the disassembled crank, and I do not see a spigiot for the rabbet so the must have done without.

Anyway, since the shim is behind the bearing, it has to be installed before the bearing is shrunk into the case. And since the traditional method involves some trial and error, the shim (and bearing) needs to be removed/installed several times until one gets the shim amount correct. I think that also answers why you cannot us the aluminum bar to just gauge the distance between the bearing races once in the crankcases.

One could divide the shims between the timing and flywheel side, but to be honest I cannot recall ever finding shims anywhere but on the flywheel side. The amount is so small, that it is neither enough to throw the rods significantly off center, or enough to correct any misalignment of the conrods. The rods rarely are exactly centered nor is it critical (allowing for the usual no rubbing, turns over freely, etc.) The flywheel side crankcase is easier to heat up to fix the bearing; easy access inside and out for the application of heat. I suppose that is why the shims are installed on that side.

-Doug 

[fix typo. 14Nov19. Doug]
« Last Edit: 15 Nov 2019 at 01:38 by Doug »

Offline Aero

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Re: Douglas Aero 1937
« Reply #273 on: 14 Nov 2019 at 20:57 »
I used 2 x 0.3mm shims to get my endfloat correct, in conjunction with 0.25mm thick paper gaskets between the crankcase halves.
Might be a good starting point?
PS that is in conjunction with the 2 oil slingers, which are each 1mm thick

Offline Eric S

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Re: Douglas Aero 1937
« Reply #274 on: Yesterday at 12:09 »
I got a bar of aluminum and will turn a tool next week.
so if I got it well we should have :
Shim - Bearing - oil slinger - crankshaft - oil slinger - bearing - shim (if needed)
I had something "different"

 

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