MUNCIE 4 SPEED INPUT SHAFT SHIM SET, PACK, ASSORTMENT, KIT M20 M21 M22 M23 Precisely adjust clearance between the input shaft and 3-4 synchronizer assembly with these shims. For accurate spacing, choose materials such as steel which are hard enough to resist scratching and deforming. Softer shims, like copper or aluminum, are typically used as wear plates between components that rub together, preventing damage to equipment. Carbon steel shims have excellent strength, but aren't as corrosion resistant as other shims.