Did a bit of cleaning. A light touch with my bench grinder/wire brush revealed McCoy 35. Just like one posted earlier except the head is plain aluminum and not black. Is this engine very rare? Fired up, turning a 10-6 prop and 5% nitro, I thought it was a 40. Seems like it would be great on a Ringmaster.
If it looks like the photo in Air Miniatry's post , with the "McCoy 35" embossed in the side of the mounting lug and the squarish main case, that is a series 21 Testors/McCoy .35. They do run pretty well, but are a bit heavy/ They are a ringed engine, with a L shaped ring at the top of the piston, so that compression tends tp help expand the ring. If yours has been sitting around, the ring may be stuck in the groove a bit. Locate a new ring to have handy, see if you can find some end gap specification for the ring gap and check that, and then you might want to take it apart to clear out the groove if you are up to it. It's not too hard, just takes a slight touch to get one side of the ring out and work it up and around the rest of the way until it's all out. Then take something sharp, and some oil, and scrape the groove all the way around, and clean it with some solvent afterwards. Clean up the cylinder, and try to put a cross hatch pattern in the cylinder walls with some 600 wet or dry paper, but don't get too aggressive. If you have a small hone with some fine stones, you can do that also but do it by hand. Clean it all up afterwards with solvent to get the metal and grit out. Replace any gaskets that may need replacing. Then replace the ring. When putting the cylinder back on, try to have the end gap on the ring in a place towards the back of the cylinder where it is smooth metal all the way from top to the bottom. Oil the cylinder walls up with some light oil and slowly work it down, and compress the ring with your fingernail or an o-ring pick. It the end gap isn't lined up correctly, make adjustments now and start over. When you are happy, put the head and screws back in, and see how it flips over. If you changed the ring, and have the gap correct, it will take some rich runs to seat the ring, and you can tell that by how much black gunk comes out of the exhaust. USe fuel with at least half castor at at least 22 to 28% total oil. When the exhaust starts to get clear, then you are there. Usually take 4 to 6 runs and do it with a smaller diameter and flatter pitch prop than what you would fly the model with. Some don't care for the big case and that is how the trend of swapping out the cylinder parts from the 21 to the Lightning bolt started. The engine isn't rare, it's probably just had the head cleaned by some one with a wire wheel like you did to the case, it happens. If it's all you have, then just run it correctly and you will get some good life out of it. Try 10-5, 10-6, and 10.5-5 props and experiment.
Type at you later,
Dan McEntee