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Posted By: Westvan71 Your thoughts on a 12-bolt rear axle? - October 19th 2010 1:27 pm
Want to know what are your thoughts about a GM 12-bolt rear axle? I saw one under a '75 Chevy van at the local yard. Did went under the van and counted the bolts - yup, it's a 12 bolt cap. I've been hunting for a good one to rebuilt to swap under my van.

I'm interested mainly for highway travel and to tow (if needed), so what would be the ideal rear gears to have?
Posted By: Mr Truckin Re: Your thoughts on a 12-bolt rear axle? - October 19th 2010 1:52 pm
i have a rebuilt 12 bolt posi with 50 miles on it.it has 456 gears
Posted By: Bullet Re: Your thoughts on a 12-bolt rear axle? - October 20th 2010 12:07 pm
The 12 bolt is a fine, strong rear. If you can get it reasonable, go for it. Having said that, and after much experience with drag cars and the like. If you have the power and/or are towing loads that will kill a 10 bolt, a 12 bolt will only last a little longer. I have a good friend with a Nova drag car that is turning 8 grand out of the hole, he has a 427 small block and it absolutely punishes everything in the car. The rear is a standard 10 bolt with factory posi with moser axles, steel caps, ARP fasteners, and heat treated spider gears. It has somewhere in the neighborhood of 7-800 passes on it and has never even had the cover off. With a few improvement parts, the rear end you have will handle anything you throw at it.
Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 20th 2010 1:37 pm
Have found a source that I wrote down the ratio codes to use while I go to the yard to pull out the rear axle. I'll remove the cap and have a look inside. I'm hoping for either 3.08 or 3.42 gears - these are good highway numbers. Not looking for high numbers as I'm not racing my van at all.

The engine would be basically stock with a RV cam throw in. The towing load would be 4-5,000 lbs gross max which isn't gonna be used often. Well, tomorrow I should know more on the junkyard axle.

Wookee, thanks for the tips, I should tear down the axle complete, check everything and re-assemble with good pieces and new parts for a good strong axle.
Posted By: wookee Re: Your thoughts on a 12-bolt rear axle? - October 20th 2010 2:02 pm
Westvan.on a GM rear 10 or 12 bolt all you have to pull is the cover.

[Linked Image]

There is a series of number stamped on the outer edge of the ring gear.divied 37 into 11 and you get 3.363636
witch makes this a 3:36gear ration.

This is a fool proof way of finding out which gear ratio you have.Due to the fact these rear get swapped out so much you can't really trust the build stickers you find on the vans's body its self

A 3:08 to 3:36 geasr ratio is a good gear ration to run behind a SBC with an RV style cam and a factory 3 speed automatic trans.

the 9.66 number was the manufactured date

........Wookee


Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 20th 2010 11:46 pm
Yep, that's what I was talking about... take off the cover to check numbers. I did that few times on my Ford Mustang and couple old cars. Nice way to be 100% sure. Hmm, my other van - GMC '75 has the 10-bolt rear axle, so I ought look in what it got for gears... someday maybe.
Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 26th 2010 1:17 am
Today I got the 12-bolt rear axle from the yard. Tomorrow will prop it on jacks and take off the cover to see what I got inside for gears. Paid $88.99 in total, so it's not a bad deal. That will be my spring axle project next year - get it all cleaned out and dress up with new parts!
Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 27th 2010 12:56 am
Wrote down the numbers off the ring gear after removing the cover. The teeth looks to be good and clean and it spins smooth. What I see on ring gear says "3817779 43:14 GM 97402" ... in truth, the ratio number 43:14 was stamped in reverse as "43:14" so I'm assuming it was in error by someone at the GM factory?

Near as I can figure - it factored out to be 3.08 (rounded to the nearest next number). Did I get that right - can anyone confirm on that? Thanks. And when was the manufactured date?
Posted By: CatFish Re: Your thoughts on a 12-bolt rear axle? - October 27th 2010 1:22 am
Did it look anything like this? This is an older one from a Photobucket search.
[Linked Image]

The pick-up and 4x4 guys call it a 3.07 gear. They were made for many years 1/2 ton and 3/4 ton. Real popular 4x4 gear for light duty weekend four wheeling.

I can't help with the date, maybe someone else can decode. chevy
Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 27th 2010 1:31 am
Yes, you have the same serial number 3817779 as mine. I must had write the ratio number crossed - I was reading off by the side on my left. So it looks to be the 3.07 gear. If it was meant for light 4x4 gear - how did it get there under the '75 Chevy van?

Oh well, I suppose that gear should be good for highway driving as it'd do good on gas.

Thanks Catfish!
Posted By: wookee Re: Your thoughts on a 12-bolt rear axle? - October 27th 2010 4:58 pm
On older GM vans you could custom order you running gear, un like today.
also on the GM 12 bolts the gear raitos can be swapped out.This involves not only the gear set them selves but the carriers also
the 3:08 rear gear set I am running in my Early came out of a 79 pick up.
I went from a factory 4:10 to a newer 3:08 doing this swap.
The 12 bolt housing its self from 69 to 79 where the same.......

........Wookee
Posted By: Westvan71 Re: Your thoughts on a 12-bolt rear axle? - October 27th 2010 6:43 pm
Good to know I can swap ring gear and gears inside the 12 bolt housing. For now the 3.07 gear is good enough to use. The important thing is I now have 5 x 5 bolt pattern on my van - no more 4.75 by 5 size!

I also found out the ratio gears were stamped on September 1974 ... so it does match the age of the van where I got the axle, the dates looks to match all round by parts. The van obviously retain much of its original parts on it.

A shame that the van is so rusty - it was an old vanner's van. Someone swooped the fender emblems and what looks like corvette waving flags above the emblems. I can see the shadow lines on fenders.
Posted By: wookee Re: Your thoughts on a 12-bolt rear axle? - October 28th 2010 8:52 pm
I like the 3:07 I am currently running in my Early.
Exalent light to light performance good hi way cruising speeds and MPG arew not too bad ether........

.........Wookee
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