I have recently acquired an old OS Max .25 engine. I have had trouble finding information on older OS engines, so I'm not sure what model it is exactly. It has plain bearings (not ball), and lapped piston in a steel chamber. I took it apart for cleaning, but the engine is very clean and appears to have been run very little. The carb is of the airbleed type, but I think it's not the original carb of the engine. For one thing, it was mounted rotated 180 degrees on its axis when I got it, i.e. the needle valve to the right (same side as the exhaust), and the throttle lever to the left, getting dangerously close to the propeller in the closed position. When I put the carb back in the "normal" position, the throttle lever bangs against the exhaust port and doesn't allow the throttle to become completely closed. The airbleed hole appears to have been enlarged, and the engine's idle is very good with the airbleed screw halfway in. Overall it runs very nicely, but I'd still like to know what model it is. I think it may be a OS Max-S, although it doesn't look that old.

And, there's an inscription reading "SB" under one of the mounting tabs. Don't know if that helps.

Cletus,

It sounds like you have one of the very old O.S. Max .25's. We'd not know where you might be able to find out the exact model.

You might want to check the piston. If it has an upright "fence" on the top, you have one from the 70's or earlier. If it's a .25 FSR, then there'll be an "F" or an "SR" on it somewhere.

The letters on the bottom of the mounting flange are the code O.S. uses to identify the production batch. If your's is old enough, even they wouldn't know which batch your engine came from.

If you can post a pic, we can give you a better idea which engine you have.