12 bolt questions and opinions wanted.

[Mike P]

I’m getting ready to put some things together to take to the swap meet in a couple of weeks and one of the items is a 68-72 12 bolt.

The biggest market for it will probably be the A Body crowd but in case I get some interest from a G Body owner I want to have the correct information to give them as far as what it would take to install this.

Here is what I THINK I know about putting a 12 bolt under a G Body:

Physically the 12bolt is pretty much a bolt in with the correct aftermarket offset upper control arms.

The driveshaft will need to be shortened.

Because it is wider (I’ll have those measurements once I get it in to the shop to clean it up) you may have to run a wheel with a bit more backspacing.

The drum to drum A Body 12 bolts seem to be going in the 500-600 dollar area (with open carriers and a bit more with a Posi) which is in the same range or a bit cheaper than the 8.5 G Body rears.

So far does this sound correct?

For the guys that have a 12 bolt under a G body is this a good swap and are you happy with it? Is there anything you wish you would have known about the swap before you started it?

Thanks in advance for any responses.

 

[455rocket]

Looks like ya pretty much nailed it. Ony thing is ya have to move the spring perches. I have the 15x7's on mine and they don't sick out with a 235/60. One other thing is if you use the stock lowers the rear isn't centered in the wheel well, it's forward a little bit. There's kits out there now to take care of that.

 

[oldtinsmith]

MikeP,

I'm sure some one will ask you "how much do I shorten the drive shaft". Just tell them that each situation is different and it's best to measure for one after the install.

Doug

 

[Mike P]

“......I'm sure some one will ask you "how much do I shorten the drive shaft". Just tell them that each situation is different and it's best to measure for one after the install......”

Doug that is good and true advice.

On the bright side shortening an existing shaft is usually a lot cheaper than having a new shaft built.

 

[bigdan]

around my place getting a used 12 bolt out of a "A" body is kind of rare by our days
good luck !

 

[Mike P]

I hear you Dan, this was a pullout from a customers car about 15 years ago.

Back then they weren’t so rare and it was more convenient to build another used 12 bolt one with a Posi and gears and then throw it in so the car was only out of service ½ a day. I wouldn’t be able to do that today because of the cost or just finding one. It’s hard to find an A Body in the yards around here now and if you do they are pretty picked over.

 

[pontiacgp]

“......I'm sure some one will ask you "how much do I shorten the drive shaft". Just tell them that each situation is different and it's best to measure for one after the install......”

Doug that is good and true advice.

On the bright side shortening an existing shaft is usually a lot cheaper than having a new shaft built.

changing the tube isn't that expensive...the only extra cost is the tube itself since the shaft has to be balanced anyway and the labor to cut vs new tube is about the same. I had mine cute once and now I need to do it again I'm going with the new tube...

as for the price of the 12 bolt I was looking for an 8.5 or an 8.2 and the 12 bolts were alot more than the 500-600 range. I bought my 67 8.2 with a 3.08 posi for $600 and I was just glad to find one relatively local with the gear set up I was looking for. The 12 bolts are much rarer and they are usually 1,000 and up

here's the newest listing on the chevelle site I joined looking for my rear end..12 bolt posi rear end - $1,700.00 CDN

 

[Carlola]

My 12 bolt is the "main event" of my chassis. I got in a complete running car already had the posi. I paid $700 to have it shortened to exact g-body length and have the spring + LCA mounts taken from a stock 7.5" and put on the 12 bolt. This included stainless steel ends to seat the bearings. Custom axles from Dutchman ran about $500 and i raised the gear ratio which was $200 for the ring and pinion. Worth every penny, its probably cheaper to buy a 9" but its nice to stay all GM.

 

[JBreu]

I dunno what car your going to be putting it in ..I have the 12 bolt in my '84 monte carlo....Depending on what transmission you have will depend on the driveshaft. If you have the stoclk th 350 then you will NOT have to cut your shaft.....You will need a conversion U-Joint for the rear of the shaft though as the current U joint will not work. You will NOT need to move the spring perches either....The rear ends are a bit wider :

68-72 Chevelle 55.250"
78-88 A/G Body 52.125"

These are flange to flange measurements.....they aren't all that much wider and you will enjoy the swap. As far as the upper and lower control arms go with UMI Performance, they have a 12 bolt swap kit, so does Icean Racing....I like the looks of UMI


Good luck 8)