The OS .91 four cycles and the .91FX (two cycle) both have a ringed piston. They are not available without a ringed piston.

One of the big differences between the .91 Surpass I and Surpass II is the carburetor. Both versions run like an OS -dependable and easy to operate- but in my opinion the carb on the Surpass II is marginally better, if only because it seems the low end needle is easier to set. Your mileage may vary here, and others may not agree, that's just what I've experienced. That said, I wouldn't turn my nose up at a Surpass I if I had an extra to put on a plane.

Also, the pumped version of the Surpass II seems to be the best idling engine I've ever run.

The .91FX was introduced sometime in the late 90's, and I don't think it has been changed since then, except for the needle valve mount.

I have flown a Four Star 60 with both the .91 Surpass II and the .91FX and I much prefer the characteristics of the plane with the FX on the nose. It was the same plane with only an engine swap, and while it flew well with either power plant, I liked the performance better with the two cycle.