Shimming

Every gear in the train sits on an axle with shims stacked above and below it to control both how deeply its teeth mesh with neighbouring gears and how much it can shift side to side. Correct shim spacing is measured in tenths of a millimetre, and it directly determines how quietly and efficiently a gearbox runs.

Because shim thickness varies by shell and gear set, shimming is done by feel and iterative testing rather than a fixed spec — spin the gears by hand, check for binding or play, add or remove a shim, repeat. It's widely considered the most skill-dependent step in a full gearbox rebuild, and poor shimming is a common cause of a "noisy" or short-lived AEG.

Feeler gauges and patience are the only real tools required, which is why shimming is often the first true internals skill a new tuner learns properly rather than skipping straight to spring and motor swaps.