[LONGHAIR]

Originally Posted by 1G1YY

I have a 68 12 Bolt diff running 3.08 gears and want to go up to 3.73. A guy has some 3.73 gears and "carrier" to suit for sale locally. The carrier is stamped 3866116. My rear is a 3 series and the carrier for sale I think is a 4 series. What I need to know is the 4 series carrier going to be compatible with my rear that has the 3 series components? Thanks for any assistance. Pete

The other responses have been correct, I would just add an comment to the last question. Yes, assuming that they are both "open" carriers and that the new one comes to you empty. The internals are the same, however, I would prefer to use the ones that were original to it. Also keep them together as a "set", once worn, mixing them is not so good.

There are likely guys who come on here and tell you how "they have done it and it's fine", but you are taking a risk. It might be, but then again might not. It's just "best practice" not to.
This is far more important with the actual ring and pinion, they get much more pressure/load applied to them.
The real issue in this swap is the used ring nd pinion themselves. That is a far bigger challenge. You would be far better off if you could actually see (still installed) or even take apart yourself, the old gears. The backlash and pinion depth are very important!
With brand new gears, there is some tolerance, they get to wear themselves into each other. Used gears have already done that. They need to go back exactly where they were, even if that doesn't meet the specs that sound ideal.