All of the Norvels have a very tight fit follow the soak down and spin break in carefully , and it takes a lot more than the instructions say, 500 turns by hand then use the starter for a few minutes and you have to keep adding oil while you are spining it, then run it very slightly rich if you run it to rich the motor runs cold and the cylinder does not expand fully and the engine runs tight and you have rod failer, also if you heat the cylinder up with a heat gun before you do any of this it will improve your chances of a proper breakin, and for the first 30 mins. of run time use atleast 5 head shims,and dont go past 1/3 throttle, this is for the average guy to do, I take mine apart and hand lap the piston to cylinder fit, then i soak it down and follow the rest of the procedure, and by the way the a/c eng. does not have the nessacary bronze bushing the heli eng. has in the nose to help with the side thrust placed on the crank end by the gear drive, and it will bind and where out very fast.