Have been playing around with spare parts and motors and it seems all the above cranks and conrods,  as well as the pistons, are interchangeable.

I have several different heads. All seem to fit fine but some have double-bubbles and I wonder if they originally came with the Perry Ports. Anybody know?

I also have an after-market finless head which I bought from Bernie Varnau at the '71 NATS. It has a single bubble, far larger than any that ST made. Bernie used them on his C35 powered Motivator IIs and took 1st in Senior Combat. His motors honked- of course I'm sure that 40% nitro helped. (See photo below from the 11/71 AAM. Dave Bush took 2nd with his Dinosaur).

While flying a pre-PP G21 on a Tyrantula this year, I felt a strange vibration at the handle. Did not fly it again and when I tore the motor down found that the rear bearing metal retainer was starting to come apart. The bearing was an ND 3L01, which was a popular replacement in those days for the stock factory ones. It had a riveted retainer versus the fold-over tabs that came on the stock bearings. I was lucky. All the balls were still intact and two small pieces of the retainer fell off in my hand. There was no other damage to the motor.

BOCA sells a high-speed rear bearing with a polymite retainer, kit number ENK-079HP, part number MR6001-2C TH9/C3. It seems to be well made. I used it and will let y'all know how things go next spring...