I have the 21C version, and this is just a heads up or FYI of what can happen, my motor has 1/2 gallon on it, and makes great power, and great run time. Last weekend, I noticed some fuel seepage behind the flywheel, no problem I'll put a TKO in. Upon pulling the head off, this is what the piston looked like, and after further tear down the whole piston is like this, crank, sleve and rod show no markings or pitting. I contacted Acend/PR-RC, and went back and forth, I'm concerned about the realiablility now, they indicated that I may have left the motor out in the open and that could have caused it, no, not true. There latest responce is " they see no problem
running in this situation". To say the least I'm not happy with it, the motor still has good compression and pinch, does run for how long who knows. I thought atleast they'd send a repalcement piston and sleeve. What do you think?