I have this engine that is called an OS Max .25. Is this the same as an OS .25 FP? I can't see any difference except for the muffler and the the lack of FP on the crankcase.

Its a "S25" made prior to the FP Judging by the muffler it dates the engine in the early 70's

The FP model is Schnuerle ported. The engine you have is loop scavenged.

The "S" came out in the mid 60's. The FP came out in 1984

The first picture is the FSR-S that came out in 77 This was a ball bearing more powerful engine. This was one of the first Schnuerle ported engine designs from OS to go into regular production.

The second picture was an intermediate model "F" in 1982 This was one of the first "ABC" designs for OS.

The third picture is the FP of 1984 but the early "FP" was still a lapped iron piston design just like your "S" model. The "ABC/ABN" type "FP" came out late in the 80's

Your engine has a iron piston w/steel sleeve and no bronze bushings on the con rod. I have the .15, .20, .30, and .35 version of your engine. You must run it on castor only fuel or it won't last. These engines aren't as rugged as the later FP series that can tolerate synthetic lube in the fuel.
They are still nice running engines as long as you don't expect the power and rpms of the later schnurle ported designs.

I have the same exact model. According to the OS site they date from 1971.

I second the use of lots of castor oil. Mine is a great runner, I love it. The #702 mufflers are a bit louder than the ones on FP's and LA's, but still quiet enough for most fields.

David

Than for the info.
I was thinking of using it on a GP combat corsair for sport flying. Not sure if it's got enough grunt though. May just pick up a cheap chinese 25 and use that.