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Would it really hurt the value or basic originality to a car if you put a M20 gearset into your m22 trans?
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Not at all. I believe any change you make to your car that is reversible is fine. In your case, I'd suggest you box up the old gears and provide them to the new owner if you should sell it. My L78 velle came with 3.73 gears in the rear, but POP and BS called out 4.10. 4.10 came with the car when I bought it. I returned them to the rear shortly after purchase as that is what I wanted.
Sam