Is the thrust washer for this engine the same as the .20-.40 FP?
Also, is the exhaust bolt hole spread (once drilled) the same as the 20-25FP, such that a .20-.25FP tongue muffler would fit it? 

Thanks in advance. I have come to own an engine that is missing the thrust washer, and, I like mufflers!


R,
Target

Mr. Target,

Not an easy answer on the thrust washer, for a couple of reasons:

1. OS changed their numbering system between the Max-S series and the later FP and FSR series. The OS 35S thrust washer is P/N 3S08 per the engine instruction sheet that came in the box.

2. Your supposition that the OS 20-40 FPs have the same thrust washer is not correct. Again per the instruction sheets. The 20 & 25 are P/N 22308000, and the 35 & 40 are 23308000.

3. I took a thrust washer from a NIB 35-S and one from a OS 25 FP. They are not identical in appearance, with the FP drive washer being longer. And when I tried to install it, it does not seat. I could probably do a bit of handwork and get things to fit--but these are not directly compatible parts. I took a thrust washer from an 40FP and it looks like it will work. Mine was a very snug fit on the diameter and the flat, but it looks like it would work. On mine, the crank pin bottomed out on the backplate with about .001" clearance still between the thrust washer and the case. Not acceptable for running. The FP washer was .005" which would get it off the backplate on my particular engine. I would check the end play very carefully before running it. You might need the steel washer or a special thickness one to keep the crank off the backplate.

4. I also note that Max-S engines do not have the steel thrust washer between the drive washer and the case. The FP series has this feature.

So, after all that, the short answer in my opinion is that yes, you can make a 40FP thrust washer work, with the caveat about the shimming.

Good luck with your engine,

Dave