Douglas Aero 1937

[Eric S]

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

[Aero]

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.

[Doug]

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

[Eric S]

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?

[Eric S]

Here are the pictures of the cylinders. 2 of the same cylinder as it came off the engine. One slightly honed.

[Aero]

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.

[Eric S]

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.

[Eric S]

Anybody could tell me if those pistons are originals?
Anydimesions available?

[Aero]

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.

[Eric S]

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.

[Aero]

Original (I assume) on the left, Hepolite on the right

[Aero]

Underside