Hot Rods Distributor shim question

I guess I've never thought about the necessity of shimming a distributor between the block and dist base. After bolting the distributor down I read recently the lower drive gear on the distributor that runs off the cam of the engine, in my case a SBC. If not shimmed it will put excessive pressure on the distributors lower gear and shortens it's life considerably.( Shims are usually needed between the base of the dist and the block besides the gasket ?

My question how does one determine the amount of shims and their thickness and if they are needed at all ?

Gene

 

Very rare you'll need a shim there, what that shim does is add some clearance into oilpump drive gear. If too long the distributor will push down on the oilpump driveshaft and pushing the oil pump gearotors into the pump plate. I stick a distr in the hole without a gasket and see if it bottoms out, if its still free then I install is proper. Sometimes you can modify a bigblock chevy oil pump in place of the smallblock and that is when you have to really check cleanances,

 

This should not typically be a problem unless a lot of other things have been done, like decking the block, heads and intake. On a SBC the cam lifts the gear to the distributor housing, where the factory washers/shims and glide plate (the funny washer with tabs) are well oiled so that there should not be any significant wear. you have to look at the fact that some current SBC engines use a plastic distributor, so clearly not a major wear area. At most, you might have to shim between the distributor base, and the intake. If you really are concerned, get some gear paint, like used to set a ring and pinion, and see where the gears mesh up at. Close to the washers, maybe a concern. More importantly, is that in excessive decking, the oil pump drive rod may become too long and push the gear against the guide plate, causing wear.

 

Typically, they're only needed if there was decking and other machining going on.

 

Exactly what OJ said.
Sometimes you know what's been done to the engine and sometimes you don't. If you don't it doesn't hurt to check. You're not checking for gear back lash or patterns, your checking so things aren't bottomed out because material were removed the block or heads. Removing that material will lower the intake and along with that goes the distributor mounting boss. Put it in without gasket, the base should land on the manifold. Hold the body down and you need to be able to lift the shaft up a few thousands of an inch.