Crank shim

[DGardner]

After 5200 miles this summer on the 94 wombat I pulled it apart for a rebuild. Before pulling the motor I could get a small amount of movement from the left crank bearing by grabbing the flywheel moving it up and down.....so I know the bearing need replaced. The surprise was after I pulled the head. There was some small pit marks in the piston but no damage to the cylinder. After pulling the motor apart I couldn't find what went thorough the motor till I pulled the crank and found that the shim on the left side of the crank had worn a bit and a very small peace at the top had broke off. Has any one seen this before? Also I don't understand why that shim is needed as the clutch when tightened pulls the crank to the right and the ignition is a tapper fit on the left. Thanks DG

[Bullfrog]

I would agree that the utility of the crank shim on the magneto side seems to have minimal value. It appears to me that it is there only to provide some assurance that the crank half will not rub on the inside of the crankcase IF the clutch nut comes loose. While fulfilling that role seems insignificant, it could be important. Without the shim it is possible that a loose clutch nut could result in severe scuffing, release of metal particles and then piston/cylinder damage. Soooo . . ., I think I will continue to be sure that the crank shim is in good condition and in place when ever I reassemble an engine.

Ed

[DGardner]

Thanks for the reply Ed.

[bchappy]

Another thought which is about as unlikely is that the right crank bearing is tied in place with a snap ring while the left is not. Maybe just in case the left bearing decided to slip to the right a little during a hot motocross race, the shim keeps it from hitting the crank. Just a thought.