This evening, I was out in the "Skunk Works" doing some R&D that might be of interest to folks who like small diesel engines. I am experienced at operating most small English and European diesels and also McCoy units but I have not until tonight try to run an OK Cub. I have a new .075 which hasn't seen fuel and recently I Ebayed a used .049.
I decided to try to start the .049 using the standard diesel fuel mix of 1 part ether, 1 part kerosene, 1 part castor and 2% amyl nitrate. No luck.... the engine would only pop when the contra-piston was fully screwed down to the max compression position.
I thought either the engine was worn out or the "O" ring was leaking. I replaced the "O" ring and still no luck. I then checked the piston/cylinder fit and found that the engine would hold compression for some time, so the fit was good.
It was obvious that the engine could not be compressed enough to run and this is because the contra-piston has a flange that limits the the amount of compression available. I'm sure Herkimer did this to keep engines from coming back with bent con rods because of the actions of overzealous ham-handed kids.
I had heard that OKs would only run on OK fuel and this may be true. I don't know what the formula of OK fuel is but it may contain a greater % of ether.
Upon removing the head and c-piston and measuring the gap at max compression, I found the gap to be a whopping .060", too much for such a short stroke engine.
What to do? I was going to make a new C-piston without a flange but got lazy and removed the flange on the stock c-piston using my lathe.
I reassembled the engine, fueled it up, flipped and compressed it until it started and it ran like a charm.

I'd like to know what other modeler's experiences are regarding the OKs.
Is the removal of the flange going to be needed on all the Cubs in order to get them to run on standard fuel?

Orv.

The only thing I can think of, is the shock pad on top of the contra-piston. If that is missing, you might not be have enough travel in the compression screw to push the contra-piston far enough. Dan Sitter usually has shock pads.

If that were the case then the screw would be bottoming out at it's head, with nothing left to push on the contra piston.  Turning the flange off the bottom of the contra piston then wouldn't fix the problem -- it'd just make it ever so slightly worse because of the missing volume of the flange.

I suspect the OK fuel had more ether, or something -- back when Diesels were new there were several different mixes (in fact, I gather that the way to tune a non-adjustable Diesel was to vary the fuel/oil ratio!).

This page has a discussion on tuning fixed-compression diesels by playing with the fuel: http://www.modelenginenews.org/ad/drone.html.

Hi Bill & Tim,
The flange is at the top of the c-piston and seats against the top of the cylinder bore preventing the c-piston from going too far into the bore and over compressing the engine. Removing this flange allows one to increase the compression more than the factory limit. Since everything was stock including the shock pad when I started and there were no compression leaks, I would have to conclude that the fuel does not have enough ether but this is just my conclusion and subject to revision if someone can give me a better answer.
Every other diesel I own has a c-piston that can be screwed down until the two pistons touch one another, The Cub is the only one I know that has a limit.
Thanks for the comments.

Orv.

That's one nice thing about John Deere starting fluid in the spray can.

Using a small plastic tube from a WD40 (or similar spray can), you can squirt a small shot of Ether directly into your fuel can, or possibly even into your fuel tank, to freshen up the fuel. 

Bill