Author Topic: Douglas Aero 1937  (Read 37232 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.