Sig Skyray

Sig also made the Skyray .35 which is a 44" wingspan.

I knew what plane it was, I was answering to Eric's comment in regards to the size

Eric, OS made a few types of .25's. The least powerful of the bunch would be the MAX .25. This can be confusing due to OS using the MAX designation on most of their engines. This engine is a baffled iron piston in a steel liner. These were also I believe the last of the strap on muffler styled engines.
How you prop the engine is according on how you want to fly. While a bunch on the internet make certain claims to how overpowering a FP .25 is for a nostalgic profile, we fly them all the time with these engines. Don't use more than a 4 pitch prop and let the engine just spin away. My foamie combat wings are 460 square inches at about 26 ounces, a FP .25 pulls those planes around with authority. However, I can fly them in a stunt like manner if need be equally. These engines can also be easily regulated by the venturi size. You want more power, open it up, and vice versa.

The early late 60's-70's MAX-S engines were very well built engines for the price. As times became more competitive, OS introduced the FSR engines. These in my opinion were the best engines OS every made. True chrome liners, high quality produced engines. The FP series even though were extremely well made, didn't quite offer the power of the FSR line. OS offered the SF and SR version 25's in and around the same time of introduction and they were essentially built around the FSR design. The FP and LA engines are what I was referring to in terms of flyability. Several parts interchange and both engines have very close similarities. One problem with the FP and LA engines is when mounted on a profile, the wrist pin end pads wear out and this could cause some serious scoring in the cylinder if not watched. Those end pads are not available unless you buy the whole wrist pin. In the winter, one of our members makes these end pads from graphite impregnated Delrin which lasts longer than the Teflon pads. However, he hasn't done so lately. I have growing concerns for a few of mine.

Most OS MAX parts are long out of production. My suggestion would be to try and see if a substitute would work such as a LA or FP wrist pin end pad. If not, buying Teflon dowel rod and turning your own is the only solution.