Well here it is, 10.35mm bore with 16.19mm stroke. The flywheel is 41mm dia and 12.5mm thick weighing in at 117gms. As you can see from the pictures it was a pain to strip, but as I was desoldering the water jacket the head came loose, so I unscrewed it. Then carefully stripped the rest and cleaned each piece, finally returned to the water jacket, I had to remove the outer sleeve then cut the screws to remove the top and bottom discs. Next I removed all the crap that was stuck in the fins and cleaned the cylinder head jacket. Finally re-assembled. All that work and still don't have any idea of the maker. There is a number on one of the mounting webs 685, on the other side is a number 2. But the Web is broken. The flywheel has RMA stamped on it and a letter J above it. So mateys what's the verdict. And god alone knows if I am able to rebuild the water jacket. Cheers Colin.
Hugely long stroke, Colin and I've never seen such a long piston. I am truly astonished that you got even a fart out of it, much less a run. I'm assuming it was built as a marine engine or would you say the finned barrel of an aero engine has been turned down to fit the water jacket? A very large shouldered top to the crankcase. It really is a mystery. I can only suggest home made.
Martin Edited to say it's 1.36 cc., so a bit of a nipper.
The fins look like they haven't been touched, the home made water jacket is basically 2 brass washers with 4 screws clamping them tight to the alloy cylinder head, then a copper wrap around soldered on with 2 pipes soldered into the wrap. The fins were full of baked sludge, but luckily cleaned up with a fine needle and small swede brush. Cheers Colin.