Tony, Yes, they are used when needed. If you can't get a good lap though, I'm not sure they would help, but what the hey, try lapping with one in place and see how it works. Can't go wrong if you don't let the shim overlap itself? I'd definitely snug the axle nut up to where the shim wouldn't shift cockeyed, just loose enough to allow the drum to turn. No, wait: The keyway will scuff up the edges of the shim, no good. Hmm... lap without the backing plate?
