Shims are available in aluminum and thermo plastic despite the latter being a lighter solution its material characteristics limit the thickness of the id od measurements aluminum is used to achieve the thinnest shims as it is a more robust material rolled aluminum shim the very thinnest shims are rolled from sheet metal 254mm shims need to be so thin as they are etched down in acid to the correct tolerance and are too thin to anodise show as silver machined aluminum shim machined t