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I don't know, this one may have been answered in the past
Is there interchangeability between M10 and M20 gearboxes?
I mean, can the internals from a gearbox from a M20 car be inserted into the shell of a gearbox for M10?
My mechanic says this can be done

I guess by "shell" I mean "bell housing"

I dont believe it as simple as just swapping the bell house. If that were the case then it would be done more often.