An OS .32 should last a lot longer than 15 gallons before losing compression. Probably the reason yours did is that you "broke it in" slobbering rich, which does not allow the parts to come up to their proper operating temperature, and can thus scrub the piston and liner. I have 15 gallons through my OS .32 and it runs like new. I broke in mine by simply installing it in the heli, setting the needle rich (two turns open), starting the engine, and then closing the needle until it was just lean enough to give me crisp throttle response with lots of smoke (15% fuel). I ran it like that for the first gallon, then leaned the needle a bit to increase power while still giving me a good smoke trail. In any event, an ABC engine should never be run so rich that the engine is cool enough to touch after a run.