RE: O.S. 26 four stroke What plane?
Hello; I have 3 OS 26's, and like them very much. In the past I have used a 26 in; a speedy bee, a Pixie bipe, a GB Freedom 20, an Ace 4 20, a GP Slo-poke, an Eglet, and probably a few others that I can't remember just now. I have only seen one worn out to the point where it needs a new piston and liner. That engine must have been abused sometime in it's life (run too lean) as the liner was dull gray in two spots, and the cam had noticeable wear on the lobes, and on the thrust ball. I don't think you can change the cam bushings, so I didn't try to fix it.
My experience is that they are a very good engine, not fuel fussy, easy to tune and run, very quiet. As long as you don't try to run it too lean, it will last forever. Good luck on your build.