as far as compressibility goes, there's no real difference... if you could contain water in the area under the rocker pedestal and not have it leak out, it'd be just as compressible as steel - that said, my rocker shims are steel... or am I talking about the wrong shims again? my rocker arm spacers are aluminium, and the shims between the rocker arms and the pedestals are steel
if you really want a scare, I'm about to put a set of these in the back of my Polo - camber and toe adjustment shims - a PLASTIC SHIM with varying thickness that goes BETWEEN the stub axle and the rear suspension beam, it's 2 pieces and one rotates inside the other, line up the marks you want for the adjustment you want and bolt the stub axle back on. The 3 bolts and 1 hole you see in this picture are what mounts the stub axle, which has the wheel bearing, brake caliper mount, everything...
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