12 bolt question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1985K10 · 03-07-2008, 03:03 AM

Im about to rebuild the 12 bolt in my 77GMC(1/2ton 4x4) which has just an open diff, and I have a 12bolt from a 73-77ish 2wd that is limited slip.
Can I put the limited slip in the 4x4 rear?

Jonboy · 03-07-2008, 03:20 AM

You should be able to. I think the only difference is the axle shafts and brake drums.

454HO · 03-07-2008, 03:25 AM

If it is a Gov-Loc differential, I would not bother with swapping. From what I understand, they are very weak.

1985K10 · 03-07-2008, 08:47 AM

Ill pop it open and check it out when a little bit more snow melts off it!

Thanks!

James McClure · 03-07-2008, 07:41 PM

1985:
I'm not saying anything against 454 here OK? I run a "detroit locker" in my truck without any problem at all and the truck has over 400,000 miles on it. I spent over 30 years working for GM in "farm country" and have only had to rebuild 3 of them due to a failure of the locking mechanism. I will say this though, you will not be able to rebuild a locker at home if you blow it up and the parts aren't cheap. jim

1985K10 · 03-14-2008, 07:48 AM

I opened the 2wd 12 bolt tonight, it has the clutch type limited slip, 34ring-10pinion, or 3.40 gears.

I opened the 4x4 12 bolt im using, it was 43-14, or 3.07 gears.

Can I remove the carrier, swap ring gears, and put it in the 4x4 rear? Or will I have to setup the ring and pinion clearance again?

bagged01extcab · 03-14-2008, 07:54 AM

You will have to set up R&P clearance again.

James McClure · 03-14-2008, 02:29 PM

1985: bagged MAY be right. Swap out ring and pinion (with new crush collar) and make your measurements. If clearence is under .002" you should be ok. Test drive for noise (whine). If it's noisy you will have to take it in to GM and get them to measure pinion depth. Chances are you'll be fine. In the 33 years I did mechanical work I never had to change the pinion depth shim dimention. I've had to replace shims but always used the origional dimention. jim

03-07-2008, 03:03 AM	#1
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	12 bolt question Im about to rebuild the 12 bolt in my 77GMC(1/2ton 4x4) which has just an open diff, and I have a 12bolt from a 73-77ish 2wd that is limited slip. Can I put the limited slip in the 4x4 rear? __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

03-07-2008, 03:20 AM	#2
Jonboy Slots go on anything! Join Date: May 2003 Location: Richardson, TX Posts: 5,957	Re: 12 bolt question You should be able to. I think the only difference is the axle shafts and brake drums.

03-07-2008, 03:25 AM	#3
454HO Registered User Join Date: Apr 2005 Location: Austin, TX Posts: 1,224	Re: 12 bolt question If it is a Gov-Loc differential, I would not bother with swapping. From what I understand, they are very weak. __________________ - Greg

03-07-2008, 08:47 AM	#4
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	Re: 12 bolt question Ill pop it open and check it out when a little bit more snow melts off it! Thanks! __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

03-07-2008, 07:41 PM	#5
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: 12 bolt question 1985: I'm not saying anything against 454 here OK? I run a "detroit locker" in my truck without any problem at all and the truck has over 400,000 miles on it. I spent over 30 years working for GM in "farm country" and have only had to rebuild 3 of them due to a failure of the locking mechanism. I will say this though, you will not be able to rebuild a locker at home if you blow it up and the parts aren't cheap. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

03-14-2008, 07:48 AM	#6
1985K10 Registered User Join Date: Oct 2003 Location: CT Posts: 315	Re: 12 bolt question I opened the 2wd 12 bolt tonight, it has the clutch type limited slip, 34ring-10pinion, or 3.40 gears. I opened the 4x4 12 bolt im using, it was 43-14, or 3.07 gears. Can I remove the carrier, swap ring gears, and put it in the 4x4 rear? Or will I have to setup the ring and pinion clearance again? __________________ Anthony 07 Tahoe LT 70 Chevelle 450HP 383-TH400 77 GMC Royal Sierra~Resto in progress~

03-14-2008, 07:54 AM	#7
bagged01extcab Registered User Join Date: Jun 2004 Location: Longview tx Posts: 270	Re: 12 bolt question You will have to set up R&P clearance again. __________________ Trey Oney

03-14-2008, 02:29 PM	#8
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: 12 bolt question 1985: bagged MAY be right. Swap out ring and pinion (with new crush collar) and make your measurements. If clearence is under .002" you should be ok. Test drive for noise (whine). If it's noisy you will have to take it in to GM and get them to measure pinion depth. Chances are you'll be fine. In the 33 years I did mechanical work I never had to change the pinion depth shim dimention. I've had to replace shims but always used the origional dimention. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.



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