The Stovebolt Site Forums The Shop Area Driveline 57 diffs

I have a 57gmc napco 4x4.I would like to change the I think 3.90 gears for a better hiway gear. Any help on what works. It has 10 bolt diffs.Where can I get a rear pinion seal and front pinion seal for this truck.The trans has numbers B248-5-GM-3761930 on it. the shaft coming out of the trans to the transfer that seal I need as well. Any help would be great. Thanks Rick

Are you also on the Napco forums? Somebody has a list of lots of the specific Napco parts seals and stuff. I don't even know if GMC used the same front and rear axles as Chevys then. My 3.90 are fine on the highway with my 32" tall tires.

If you take the seals out any parts store can get the replacements off of the numbers on the old seals. But I like having my new parts in hand before I take something apart, so I understand if you don't want to do that.

You'll need to verify what axle is in the rear. IIRC, the GMC with the Napco conversion used the HO33 in the front, and a Spicer 45 in the rear. The only gear available for the HO33 is a 3.38 from Patricks, the Spicer might be more difficult. It's similar in construction to the Spicer/Dana 44, I've heard conflicting reports as to just how much will swap, mainly gearsets. Seals for both axles should be readily available from any parts store, as will the seal for the transmission. The transfer case seals I'm not sure about, but if you can get the original out without damaging it too much, a good parts man can measure and cross reference it.