WTF? why do I have a 12 bolt rear-end in my car?

[slowboi]

So three years ago I bought an 86 GN that was presented as 95% stock. I put it in storage while I accumulated parts to restore it. As time past I ordered a Baer big break kit, coil over suspension, sway bars, an eaton tru-trac 30 spline set up, etc etc.

I start tearing down the rear end today to start doing the suspension and BAM! 12 bolt rear.......

Was this the thing to do at one time? Swap in a 12 bolt? Im confused........

 

[Chuck Leeper]

The rear end was the 5% that was not "stock"........A pic?

 

[trailinjake]

To my knowledge, the only 12 bolt rear that will fit our car is 68-72 chevelle. I think you had to get different UCA to make it work properly

 

[pacecarta]

considering that rears wear out and the 8.5 rear for the buick is not a common item or cheap... its one of the things that should have been verified before purchase , that and the BRF tag on trans.

im sure theres lots of guys running around with 7.5 rears and olds or caddy transmissions that are ok stock but once they start leaning on the car and they give up then they discover its going to cost a lot more than they thought

so now its either find an 8.5 housing for your parts.. or post a parts for sale listing.

oh and an FYI , because the 12bolt housing upper ears are wider apart on the A body , it uses specific offset upper control arms for fitting the Gbody ... so if you go to an 8.5 those will need replacing , i forget if the driveshaft worked as is or required a conversion ujoint swap at the pinion

 

[lrayzer]

Other than a pile of parts that don't fit, the 12 bolt is a valid rear for performance, depending on your end goal, cornering, drag racing, etc you may want to keep it, you can get a brand new bolt in unit from Moser, in 12 bolt or 9" Ford ( shudder)
Would be easy enough to sell the unit and find a 8.5"

 

[trailinjake]

Other than a pile of parts that don't fit, the 12 bolt is a valid rear for performance, depending on your end goal, cornering, drag racing, etc you may want to keep it, you can get a brand new bolt in unit from Moser, in 12 bolt or 9" Ford ( shudder)
Would be easy enough to sell the unit and find a 8.5"

I agree. These 12 bolts are plenty strong if set up properly. 8.5 are getting really hard to find at a reasonable price. Almost better off buying a new 9"

 

[novaderrik]

yes, a 12 bolt swap into G bodies used to be the thing to do. someone that didn't know any better might have thought that the GN had a 7.5 like every other G body and swapped it in..
he has a 12 bolt... he should keep it..it has a few things going for it that makes it better than an 8.5- slightly bigger ring gear and 30 spline axles among them.
maybe do some digging into it to see if it's still got c clips holding the axles in or the pressed on bearings that are installed to eliminate them.

 

[1980monteturbo]

oh and an FYI , because the 12bolt housing upper ears are wider apart on the A body , it uses specific offset upper control arms for fitting the Gbody ... so if you go to an 8.5 those will need replacing , i forget if the driveshaft worked as is or required a conversion ujoint swap at the pinion

good point
just to add - I used adj upper control arms with the ball joint type end fittings . this allowed the uppers to swivel a bit and fit into the ears and not bind.
Also I had to adjust the length to as short as the threads would go to get the correct drive shaft / pinion angle something like 3 deg iir.
I think a company called South Side Machine ??? made a g-body conversion kit years ago that had a shortened dog legged upper to fit the wider ear location on the 12 bolt
The width of the stock 12-b A-body rear is wider than the g-body 10-b . I can see it on my car. I forget the exact lenghts but eyeing it could be approx 3/4 in or so on each side. Attn to tire / rim offset for wide tires required.
I had a custom driveshaft made since the trans and rear on the mc was replaced but when i did the research ( a decade ago ) there is a difference with 12B . Shaft should be shortened a tad. The risk is the stock lenght shaft end will bottom out in the trans if you hit a large bump.
link : photo of 12bolt and adj uppers in my monte

 

[slowboi]

 

[slowboi]

I think Ill be getting rid of it, I have a huge amount of parts for the 8.5 assembly already bought. Baer rear disks, complete coil-over ride tech suspension, not to mention a new tru-trac 30 spline set up and axels......

The BFR trans is there, just didnt think to check the rear end when I bough this thing.

 

[getdowngranny]

If you plan to sell it, it should be easy to sell. Someone took the time and money to upgrade the posi unit and the axles on that 12 bolt. It should hold enough power on the street. I had one on my 87 cutlass with a 450hp 377 sbc and held up just fine. It was a stock posi unit with stock axles.

 

[ttaowner]

can you take some betterpics of that housing it almost appears to me as being aftermarket. the main caps and bolts don't look stock either. nobody ever seems to wire lock the bolts either like yours is done. my opinion it might be it a entire aftermarket rear

 

[forcefed3.8's]

Why is everybody acting like the 12 bolt is a downgrade from the 8.5, saying they hold up on the street, etc? It is an upgrade for sure even stock vs. stock. Not needed, but it isn't something that has to be be sold. You have a strong 12 bolt with aftermarket parts, better than an 8.5 with aftermarket parts.

 

[jopaustu]

Some how I ended up with a 12 bolt rear end cover. It has either GS Or Buick stamped on it (stored in attic for years). Maybe a poston cover. Have no use for it. If by chance you are interested its available

 

[87_we4]

I'd trade my 8.5 for that 12 bolt

 

[TURBOPOWERED68]

How many bolts on the ring gear?

 

[GNVenom]

"12 bolt" refers to the bolts that hold the ring gear onto the carrier, not the rear cover to the differential housing.

 

[slowboi]

"12 bolt" refers to the bolts that hold the ring gear onto the carrier, not the rear cover to the differential housing.

Ok, i didnt know that.... My T/A performance girdled cover has 10 bolt holes, (its for the 8.5 inch GM unit)

I assumed this was a 12 bolt due to the holes.... when I have time ill go count the bolts on the ring gear....

 

[slowboi]

Also the Eaton serial number on the diff comes back as a chevy 12 bolt part number.....

 

[TURBOPOWERED68]

"12 bolt" refers to the bolts that hold the ring gear onto the carrier, not the rear cover to the differential housing.

Yes and no
There are BOP rears with 12 bolt covers and 10 bolt ring gears.

Then there's the more desirable
12 bolt 12 bolt / cover and ring gear.