way back when shims were new to most techs, sanding (or whatever method used) was thought to be okay, but as metallurgy progressed, we learned that the hardened surface of the shim is very thin, sometimes microns, and thinning the shim removed this layer. Result? shims that wore abnormally, unevenly and prematurely. We won't even discuss the whole perfectly flat thing. MHO? just swap the damn things out! They're not that expensive!
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