Originally Posted by
shane55
Lets say the crankshaft, conrod and bearing are replaced whats to stop the bearing shifting back slightly and wearing off the backplate again? What I cant understand is why there is apparently nothing to prevent this happening?
Well that is just the way they designed that engine .... right or wrong. That way typical of some of the early Chinese engines to have glaring errors in their design.
Regarding retaining the rod / bearing on the crankpin, that can be a major problem. Engines like the OS line of larger gas engines have a successful system for doing that. They use drilled and internally tapped crankpin. Into this they use a special hardened and flanged screw that screws into the crankpin from the rear and retains the rod assembly. Other engines use a flanged and hardened crankpin that is pressed into a hole in the crankweb retaining the rod to the crank as a semi-permanent assembly. Most of the cantilever crank engines use this later system. The problem is that neither of these systems can be adapted very well to an existing engine. It would be difficult if not impossible to drill and thread the crankpin, etc.
There is one thing you might possibly try ..... you can install some small washers on the wristpin inside the piston between the rod and the rear piston pin boss. This would prevent the rod from moving rearward and sliding off the crankpin. Some Chinese engines used this system .... some with success and some were great failures. You need a material that won't wear away inside the piston and that can be difficult to achieve. The washers would be contacting the steel rod on one side and the aluminum piston on the other. Hardened steel would probably be the best material. Zenoah uses such washers on some of their engines to center the rod on the crankpin between the crankshaft's (2) counterweights. Not a cantilever arrangement but the washers are there and they don't wear away over time .... so the system can work.
Good luck ..... probably easier to just replace the engine!