I have a OS 40 fp that went bad bunrt it up now I'm thinking about rebuilding it I checked and the cyl.and piston will cost $36.00 dollars .with buying the parts and the cost of the engine i will spend about $90.00 in it is it worth it or should i just throw it away.

Keep it for parts and maybe get a better one off ebay. Or you could find something that is more powerful like a Thunder Tiger GP42. They are only $62.99 from Tower Hobby.

If the cost to rebuild is more than 50% of the replacement cost
then you are usually better off replacing.
This is the "rule of thumb" I and many others use.
Good Luck,
KW_Counter

HI,
I second the keep it but get another off ebay. There are a couple for sale now I noticed.

There's one right now on RCU buy and sell forum for $25.00 plus shipping. Assuming it's good, you won't beat that deal.