I've been running TTpro .40's with 9/6 props for the last few years, they run around 16,500 or a little more on the ground.

I've had rear bearings fail, no warning, no corrosion, no rust - the stamped metal ball retainer cage gives up.

This pretty much makes hamburger out of the piston, sleeve, head, and sometimes the case.

Now ALL of my TTpro .40's get new bearings with plastic ball retainers in the rear bearing, before they ever run. The bearings will probably be replaced at some point before they're worn out, and I haven't seen that make much of a difference in piston/sleeve fit anyway. After they're past their prime for racing, they make good sport engines still!