I have an O.S. .61FX and I love it. I would rather have gone with the .50SX only because it puts out 1/10 less hp and is alot less heavier. Ive used Magnum, MDS, K&B and even Dinamite for cars. Ive used a lot of different engines and I can honestly say, besides saito, OS is the only engine I will ever buy. Ive never had a problem with any of them...