Originally Posted by
Old Fart
Well i hope the thin gasket solves the rubbing problem.Can anyone tell me the reason for going to an aluminium backing plate?
I have two 82's and they both have a few years and lots of hours on them as they are my favourite engine for 46 size decathlons.During numerous tear downs to replace bent crankshafts,bearings and collateral damage like bent intake tubes,i've had no problems with the plastic backplates.You can see light witness marks on the inside of the backplate from the crankpin and perhaps the plastic is better in this regard..
My FA150 had rub marks on the backplate but they weren't from the crank pin. They were from the connecting rod & there was a copposponding shiney rub mark in the big end of the conrod. The rub mark on the backplate was not concentric like it would be if the crank pin was rubbing. My guess is that the conrod was guided forward by the piston @ the top of the stroke while the rod was able to float to the rear near the bottom.
The purpose of the close fitting backplate is to keep the conrod from floating too far tom the rear & putting side load on the piston. Is that absolutely necessary? Probably not, but you know how the Japanese are about precision.