What's the purpose of shimming a crank?

[francisjohn]

I've read shimming the crank to prevent lateral play. What is the point/long term benefit? I've put quite a bit of time into my build, and probably wouldn't if I ran just a "stock" china doll.

The only conclusion I can come up with is that it puts an even load on both bearings. I suppose is there any anticipated preformance benefit? or does it prevent wear, and if it does prevent wear then where?

How do you calculate how much to shim? My idea would be to push the crank to one side, let's say the magneto side. I then measure on the opposite side how much of the crank is sticking out. Then, push the crank laterally towards the small bevel gear side, and measure the difference. If there is a 1mm difference, then shim it 0.5mm on each side. This is a hypothetical number.

When I pressed new case bearings, I thought maybe it might fix the lateral play, however upon measuring the stock bearings, they fit standard 6202 bearing measurements. It appears this is good ol' China.

 

[GW's Motorized]

Shimming just helps prevent wear, especially on higher performance engines

 

[francisjohn]

Shimming just helps prevent wear, especially on higher performance engines

Wear on the crank or the bearings?

 

[GW's Motorized]

Wear on the crank or the bearings?

Both, as well as the rod and sometimes even the piston. Slop in the crank can cause sideways pressure on the rod and piston

 

[Wrench]

Inexpensive M/B engines from China come with loose tolerances
If the Crank don't have the proper "End Play" you won't have a well balanced engine and it'll lead to unnecessary wear
Wear or failures & loss of power (friction) depends on how great the end play is, That's why there is tolerances

Normal Crank End Play should be between .003" and .006" but these 2 stroke CG engines can probably get away with alot more
To measure Crank End Play,
Take the cylinder off,
Tap the crank to one side,
Pick a point of reference to measure,
Tap the crank to the other side,
Measure movement

A shim made outta the correct gauge copper sheet or even gasket material can be placed behind a crank bearing or the bearing on each side of the crank to get the end play withing tolerances along with the crank centered in the case

My 4 stroke 138cc Whizzer Crank had excessive end play of .020" and the cam gear face would rub on the crank weight face (friction)
I made a .016" copper shim and put it behind the RH crank bearing This put the Crank End Play @ .004" perfect

IDK how to shim a 2 stroke CG crank