I have to agree with a lot of the above suggestions. On the 51, a AX makes sense. I don't know, but I"ve never disliked the the plain bearing TT GP42's that I've had. Similar to the OS LA 40. I think they have great purpose and on something a little smaller, they would be just great. My first trainer was a Kadet Senior that I put a TT GP42 on it. Not great power, but I had a blast with it. So it really comes down to how much you can spend and what you're putting it on. Nothing wrong with choosing a OS 40 AX.