83 12 Bolt Posi in a 72?

[ClassicIron]

Hoping someone can help me figure this one out. I found a rear end for sale said to be from an 83 c10 with 12 bolt posi and 3.07 gears. Should it be compatible if I were to swap the diff into my 72 1/2 ton rear end? Stock 12 bolt with 3.07 open diff and 30 spline axles. New posi units are typically described as being 64-82 but I haven’t found what the difference is with 83+. Thanks in advance!

[ClassicIron]

Pic for reference

[Romo912]

Originally Posted by ClassicIron

Pic for reference

I also have a question. I have a 67 gmc leaf springs and I’m wanting to swap out the 12 bolt for a 80’s 10bolt . I’m thinking it should mount right up but I really don’t know. I’m hopeful that someone has done this and can share some wisdom. Thanks
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[Palf70Step]

Far as I know they dhould work as long as you use everything from thenewer unit. As long as they are both truck rears, you should be good. Car to truck are not all interchangeable but I do not know much about car 12bolts.

[pjmoreland]

I wonder if that is actually 1983 rear end. The Weld looks suspect, so maybe it was swapped in from an older truck. Is there a date code on the pumpkin casting similar to the one in this photo. The letter is the month, the first number group is the day, and the last number is the year.

[cwcarpenter98]

If the reference pic is the 12 bolt you're looking to purchase, yes, the posi should work in your stock 72 rear

[57taskforce]

I don’t think the 12 bolt truck rear was available past 82. They began phasing out in the late 70’s/very early 80’s in favor of the 8.5” 10 bolt and 9.5” 14 bolt SF. Someone likely swapped that one in at some point. My ‘57 is on an ‘88 K5 (yea I know it changed to the R/V series by then) frame and it also had a 12 bolt swapped into it as well from a mid 70’s blazer. It’s been that way since I bought it in ‘05.

[MikeB]

I didn't think 12-bolts were available in trucks newer than 1972. Later 1/2 ton trucks used the 8.5" 10-bolt as standard.

Well, I see that Rock Auto sells several parts and kits for 1973 8.875" diffs (12-bolt I assume), so it must have been standard on trucks with higher GVWR.

Your biggest issue, IMO, is the type of springs used -- leaf or coil.

[ClassicIron]

Thanks all! It's making sense now.

[Tomchev]

Trucks used the 12 bolt through 1982. The pinon in the 12 bolt truck was smaller than the 12 bolt car.

[MikeB]

Originally Posted by Tomchev

Trucks used the 12 bolt through 1982.

You're right. I just read in my 1982 shop manual that 10-30 series trucks used 7 different rear axles, categorized by ring gear diameter. They were 8.5" (10-bolt), 8.875" (12-bolt), and all the way up to a DANA 10.5".

My 82 C10 had the 8.5" 10-bolt. I suspect a step-up or two in GVWR got you the 12-bolt as part of the package. Maybe even in a C10.

[Ironangel]

Originally Posted by ClassicIron

Hoping someone can help me figure this one out. I found a rear end for sale said to be from an 83 c10 with 12 bolt posi and 3.07 gears. Should it be compatible if I were to swap the diff into my 72 1/2 ton rear end? Stock 12 bolt with 3.07 open diff and 30 spline axles. New posi units are typically described as being 64-82 but I haven’t found what the difference is with 83+. Thanks in advance!

The axle in the pic is a Truck 12-bolt, the carrier is a factory posi and everything should swap over to your axle housing. You will likely need a shim kit to set the correct backlash after you transfer your ring gear to the posi carrier. If I were you I'd go ahead and replace the carrier bearings and races on the posi unit. Keep track of the shim packs, there are either two factory cast iron shims or two stacks of thin shims, one on each side of the carrier. Here's a small installation procedure, https://www.randysworldwide.com/web_...l_Complete.pdf If you dont disturb the pinion gear and nut on your axle you should be ok to just put your ring gear on the posi carrier and install it per the instructions. Ring gears and pinion gears are mated pairs, they are lapped together from the foundry and should never be split up...Good luck, ask lots of questions...

[MikeB]

Originally Posted by Ironangel

...everything should swap over to your axle housing.

I read through his first post too fast, and didn't catch that all he wants to do is swap in the posi carrier! I would add that he should check the clutch discs for wear.

Good tip on keeping ring and pinion pairs together.

[Ironangel]

Originally Posted by MikeB

I read through his first post too fast, and didn't catch that all he wants to do is swap in the posi carrier! I would add that he should check the clutch discs for wear.

Good tip on keeping ring and pinion pairs together.

I agree, I didn't want to add to the confusion by having him dismantle a posi unit. There were so many configurations of clutch discs it's hard to say how many of what kind of frictions and steels that unit has. The test for a posi is done when the diff is fully assembled and the axles and wheels are on. Jack up one side and measure the torque it takes to release that wheel...I'd just look it over good, make sure no discs are missing or broken. Check the springs and plates for breaks or cracks. Use a pair of Channel Lock pliars to compress the spring plates...

[MikeB]

Originally Posted by Ironangel

The test for a posi is done when the diff is fully assembled and the axles and wheels are on. Jack up one side and measure the torque it takes to release that wheel...I'd just look it over good, make sure no discs are missing or broken. Check the springs and plates for breaks or cracks. Use a pair of Channel Lock pliars to compress the spring plates...

That's a good way to do it. The only posi I ever tore into was a Ford Trac-Lok. And the final test was the work bench version of just what you said about measuring torque. I've been waiting 4 years to see my work in action. It's for a friend's 56 Chevy car that is turning out to be a perpetual project car!

My truck's 12-bolt has an Auburn cone-type diff that I installed over 20 years ago. It still lays down two strips of rubber, and doesn't chatter in turns, but doesn't really get much abuse.

[57taskforce]

It’s not going to be bolt in, the spring perch widths are different between the two as well as the shock mount locations. You will have to cut the old perches and shock mounts off and weld new ones on.