I have this motor on all 6 of my 40 size planes, from day 1 I have never had any problems. It's easy to tune and has lots of power.......Now with that said I decided to try a new motor from Jett Engineering same bolt pattern as Os 46 and Os 55, but costs almost twice as much as the 55Ax. The power it produces is just scary...But Os has been my favorite motors since 1994..