Planning on swapping in a set of good used 3.55 gears into a 70 B body 8 1/4 axle which has 2.71’s in it now. This axle assembly will be going in a 52 plymouth powered with a flat head six. So the 8 1/4 will be more than stout enough for the end use.

I’ve not rebuilt an 8 1/4 before, done 8 3/4’s....but not changed the gear.

Figure I will need to replace bearings and so on.

Maybe a stupid question to those in the know, be kind. My question is about the pinion gear shim(s) and setting the pinion depth. The donor 3.55 set has the old bearing still on it. Will the shims on it when used with a new bearing get close to being the right depth for the pinion or are the 2.71 shims a better bet ?

yes start with the shim that's on the pinion now, assemble it and see how the pattern looks adjust from there. If you,ve done an 8 3/4 you can do an 8 1/4

I take a small sanding roll on a die grinder and sand out the bore on the new bearing to make it a ton easier to change shims if necessary. Also, if you get the long carrier spanner tool that goes through the axle tube life gets easier too.

I agree with slant, make some "setup bearings' out of the old ones by boring them out to a slip fit.

Ok cool. I had read about opening up the ID of the bearing to make checking the depth easier and swap shims.

A buddy of mine used to have a home made tool that would hold the companion flange on the pinion for tightening the pinion nut. He moved away and I no longer have access to that tool. Any ideas about where you would get one ?