They use a standard 2.00″ by 1/8″ piston ring. As Tom has pointed out, Dave Reed at Otto Gas Engine Works should be able to help you.
Here’s what I do to free stuck piston rings. Carbon stuck or seized by rust, it has never failed to work. If you soaked the ring grooves in whatever penetrant you like, then you heat the piston crown with a propane torch. Don’t heat the skirt or the rings themselves. get the crown good and hot then use a piece of hardwood (not pine or anything soft) and start tapping on the ring in the gap area. Tap for as long as it takes to see any movement whatsoever. it might take 5 minutes, it might take half an hour, just remember that time is what you have but spare rings is what you don’t have any of. You will see some movement on the opposite side of the ring from which you’re tapping. Once you see it, move to the area that moved and tap it back to where it was. reheat piston as necessary and don’t be afraid to wear a welding glove to hold it if thats what it takes. Sometimes I cheat and clamp the piston in a vise between two pieces of 2 X 4 and tap back and forth perpendicular to the vise jaws (not parallel to them). Heat, time and vibration will get rings to walk their way out of grooves you never thought possible. Re-heat and re-oil as necessary. I also only use Gibbs penetrating oil in my shop. I also use WD-40 but only as a cleaner because its a better degreaser than it is a penetrant or lubricant. Eventually you will see the ring start to work its way out. When one section moves, tap it to push it back and the tension you’re putting on it will be spread around the ring, and like a short circuit, the area where the force finds the least resistance is where it will exert itself and cause the greatest amount of motion. Slow motion, sure, but motion nonetheless. I have honestly achieved a 100% success rate freeing up stuck rings this way. I’ve even reclaimed the thin rings on a Mercury piston, so it can be done. I’ll post a picture of an Evinrude piston that I got the original ring out of. Wait till you see it…
PM T2
EDIT – As promised, here are images of the piston I extracted from its bore, then loosened the remaining piston ring that wasn’t broken. For the record, the ring was already broken by the time I got the piston out.
As you can see, the rust pitting is quite heavy. The intact ring was rusted into the groove. It took about a half-hour to get it freed up. Granted its a big wide ring which may or may not have helped, but the point is it was badly rusted and still came out in one piece.
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...