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Old 05-14-2017, 06:01 PM   #1
87Flh
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10 bolt to 12 bolt swap

I have an 87 R10 with a 10 bolt rear, I want to swap it out for a 12 bolt. Will any rear end from 73 to 87 interchange? If I find a 12 bolt from an earlier truck, will it be a direct bolt-in for my 87? Thanks.
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Old 05-14-2017, 08:01 PM   #2
69heavychevy
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Re: 10 bolt to 12 bolt swap

Yes its a bolt in swap, but you will need a conversion u-joint.
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Old 05-14-2017, 08:11 PM   #3
bbbc10
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Re: 10 bolt to 12 bolt swap

You have to worry bout how wide the rear end is from diff models just cant bolt it in an go
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Old 05-14-2017, 08:12 PM   #4
87Flh
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Re: 10 bolt to 12 bolt swap

Thank you sir. I thought that was the case, just needed confirmation from somebody that knows these trucks better than me. Thanks again.
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Old 05-14-2017, 08:13 PM   #5
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Re: 10 bolt to 12 bolt swap

Are they generally wider than the 10 bolts?
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Old 05-14-2017, 09:20 PM   #6
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Re: 10 bolt to 12 bolt swap

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Originally Posted by bbbc10 View Post
You have to worry bout how wide the rear end is from diff models just cant bolt it in an go
Explain please ? It sounds like you're saying that 73-87 there were different widths.
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Old 05-14-2017, 11:28 PM   #7
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Re: 10 bolt to 12 bolt swap

I've wondered about this swap as well (but in the opposite direction). Would anyone know if the driveshaft length is the same between the two?
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Old 05-15-2017, 08:08 AM   #8
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Re: 10 bolt to 12 bolt swap

I just grabed one. Mine hasn't seen the road yeat but it is bagged and i haven't had any problems
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Old 05-15-2017, 08:25 AM   #9
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Re: 10 bolt to 12 bolt swap

I was under the impression that they were the same widths as well.
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Old 05-15-2017, 02:26 PM   #10
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Re: 10 bolt to 12 bolt swap

autozone part# 3-3130 might be the combo ujoint you need. i ended up putting a 77 drivetrain into a 84 k5 blazer, used everything but the 12 bolt rear end so it might work backwards?

it gets kind difficult telling parts store guys that you need a universal universal joint so you can try that number and see if its the one you need
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Old 05-15-2017, 06:53 PM   #11
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Re: 10 bolt to 12 bolt swap

2wd vs 4wd might be different width, I know it is for 88-98's. That and the u joint, otherwise they bolt right in. Not really much stronger then the oem 10bolt though so I'm not sure the point besides bragging rights?
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Old 05-15-2017, 09:38 PM   #12
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Re: 10 bolt to 12 bolt swap

Tom nailed it on the head. The truck 12 bolt has a pinion shaft diameter of 1.438 and the 8.5 10 bolt has a pinion shaft diameter of 1.625 leading many to argue that the 8.5 10 bolt is actually stronger. I've had a 8.5 10 bolt that lived (a long life) in a low 10 second full steel body malibu that made over 700 from a stroked big block that also made crazy torque. I don't believe that going from a 8.5 10 bolt to a 12 bolt truck rear end is an upgrade. There is a reason that GM is still running basically the same rear in ever 1/2 ton vehicle they build.

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Old 05-15-2017, 09:38 PM   #13
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Re: 10 bolt to 12 bolt swap

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Originally Posted by oldpurple View Post
I've wondered about this swap as well (but in the opposite direction). Would anyone know if the driveshaft length is the same between the two?
They should be, I used a driveshaft out of a swb with a 10 bolt and used it on my 12 bolt. Donor truck also had a th350.
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Old 05-16-2017, 11:28 AM   #14
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Re: 10 bolt to 12 bolt swap

At this point I am just planning ahead, I want to change my gears and install a posi. The reason I am considering a 12 bolt is because a friend has a posi for one. I think I will be better off keeping my 10 bolt.
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Old 05-19-2017, 09:57 PM   #15
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Re: 10 bolt to 12 bolt swap

Currently doing this swap, 10 to 12 bolt in my wife's 86. Had a good 12 bolt with a 3.08 posi (good eaton not a gov loc). Decided it was cheaper to put 3.73s in the 12 bolt than to buy a posi and gears to replace the 2.73s in the open 10 bolt.
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Old 05-20-2017, 09:02 AM   #16
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Re: 10 bolt to 12 bolt swap

The 12 bolt has a carrier break between 3.42:1 and 3.73:1, so you won't be able to use the eaton posi with 3.73 gears. That's another area where the 10 bolt is better. There is no carrier break for the 8.5.

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Old 05-20-2017, 10:01 AM   #17
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Re: 10 bolt to 12 bolt swap

Quote:
Originally Posted by Mr.Chevy4x4 View Post
The 12 bolt has a carrier break between 3.42:1 and 3.73:1, so you won't be able to use the eaton posi with 3.73 gears. That's another area where the 10 bolt is better. There is no carrier break for the 8.5.

Mike
another minor issue with the 12bolt is its a little bit more rare then the 10bolt. if you snap a axle shaft you might have a little bit harder time running across one. they are slightly different in the c-clip area where as craigslist is usually littered with 10bolts
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Old 05-20-2017, 12:34 PM   #18
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Quote:
Originally Posted by Mr.Chevy4x4 View Post
The 12 bolt has a carrier break between 3.42:1 and 3.73:1, so you won't be able to use the eaton posi with 3.73 gears. That's another area where the 10 bolt is better. There is no carrier break for the 8.5.

Mike
That's why they make thick gear sets. I purchased a set from Richmond, it worked out great.
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Old Yesterday, 11:58 PM   #19
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Re: 10 bolt to 12 bolt swap

I swapped the 10 bolt in my '83 for a 12 bolt from a '79 and only needed to change the U-joint. The U joint on the 10 bolt was actually larger than the one on the 12 bolt. In hindsight, I wish I would have kept the 10 bolt or swapped in a 10 bolt from a '91 'Burb with the larger axles.
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