I'm trying to get the pinion from an A+ into an A gearbox, but the bearing retainer is a little is to small, is it possible to fit the retainer from an A+ box into the A box

Have you changed diff ratio? if you are going to a taller diff (ie smaller number like 3.7:1 to a 2.9:1) the pinion will be bigger, you can mill the retainer with a file, or a drum sander on a drill is great.

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speed costs....how fast do you want to go...