View Full Version : differential dilemna? you decide!

02-15-2002, 03:40 PM
So I'm getting into this rear on the 65 cargo van and what I find is curious:

A twelve bolt rear with a 7.5 diameter ring gear and 30 spline axle. In other words, the innards seem like a standard 10 bolt, but the housing seems like a 12 bolt. Is this a van phenomenon? Has anyone else seen this? I am trying to determine what ring to order and it all seems odd. Any help is appreciated.

Thanks again, Van gurus!

chevy van 5150
02-15-2002, 05:28 PM
The 30 spline axles are pretty common in both 10 & 12 bolt, but as far as I know 10 bolts used either a 7.5",8.2" or an 8.5" ring gear. Where the 12 bolts were all 8.875" I don't think it's possible to put a 10 bolt carrier in a 12 bolt housing, But I've never had both apart at the same time and compaired them. Maybe you have the real "Mystery Machine" ???

02-15-2002, 05:40 PM
So far, that is the truth.
According to Part number pdf download on this site, the original rear with a 3.73 ratio (which is what I have) for 64 to 66 vans is GM# 3857229. I am currently attempting to learn what that number means. heh. Thanks for the input.

02-15-2002, 09:33 PM
Erm...what bolts are you counting? A true 12 bolt has 12 bolts holding the ring gear to the diff- it's a coincidence that the inspection cover has 12 on passenger cars. Truck 12 bolt covers are actually covering 10 bolt carriers. If you look at a muscle car 12 bolt and compare it you will see that it's much larger- of course then there is the 67-70 14 bolt cover rear that has the same carrier as the 1 ton dually rear.....

02-16-2002, 12:45 AM
Yep Yep, I waded through the 12 bolt inspection vs. true 12 bolt scenario already. Here is another question... I was told the proper way to measure the diameter of the ring gear is from center of the teeth on one side to center of the teeth on the other. Then someone else just told me it is just from outside edge to outside edge. Can anyone confirm which is correct?

Thanks for the help so far!

02-16-2002, 07:08 PM
Good question! My experience has only been to verify what diff I have, since I usually just have a local shop do the rebuild, I've never had to measure the ring gear. By the way, I STRONGLY recommend having a competent shop do this work, unless you have all the right tools and indicators. Go to a car show or cruise night and talk to the guys who've hot rodded their cars and see who they recommend in your area. If you get a dozen or so referrals to one shop, you can bet they are good.

02-16-2002, 11:17 PM
I will definately have someone put the new unit in and do the rebuild. i just don't trust myself and my lack of tools to properly set the pinion depth, etc.

I'm just trying to do as much as possible to minimize the cost of this $$$$ repair.

Thanks for the input again.