Crank pin.....
I have been running a 1st gen BRB set for a long while now.....the crankpin on the crankshaft have worn away and that the flywheel will rock back and forth at TDC. Initially, i thought it was the conrod brushing, coz i did not get any play when the brb set was new, but it turn out 2 b the crank pin !!! Replaced the crankshaft from the back up motor......
Cya
philip