Proper shimming is necessary. Most of the times I do my shim job with almost nothing else in the gearbox, and I note how much shims I need on each side of the gears. Then I make a first check by putting the 3 gears in, and tightening the gearbox without other parts. If the gears run smoothly, and have minimal (but do have) side to side play, then I put everything back in and check the GB.
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