I'm putting together a little T-6 pylon racer and was given an OS Max 25 engine. I understand it needs to be the non-BB type (bushing) for competition. I'm having a heck of a time trying to it the engine. It's stamped with the following:

OS Max (on cylinder left side)
25 (on cylinder right)
KP (under right mounting tab)
2607 (on case plug)

I've tried the OS site - no help. So here I am. Thanks for any help!

Chuck Gould

could the KP actually be an FP ?

Nope. I just double-checked under a mag. and it's "KP". Maybe I'll post a couple of pics tomorrow.

the easiest is to pull the prop drive washer- all the OS25s use a slip-on drive washer that will come right off. if it has a thick(approx .020) black steel washer behind it, and you can see a front ball bearing, it's BB. if the prop drive washer has a thin (just a few thou) stainless steel washer behind it, and you can see no front BB (you may be able to see the thin bronze bushing) it is plain bearing. another way is to remove the carb- the SF and FX BB engines use a 13mm carb neck, the plain bearing engines and the FSR BB use a 10mm- the FSR 10mm carb is dual needle, the plain bearing engines are airbleed carbs. also check the muffler bolt spacing- the FP and LA plain bearing engines use 32mm muffler bolt spacing, all the BB engines (FSR, FX, SF) use 35mm bolt spacings. the KP code on the engine lug is probably a factory lot or date code.

Great! I'll look "into" it tonight. Thanks.