GM 12 bolt ID

Tonight the new rear end for the Cutlass was dropped off. I was going to have to go pick it up this weekend but the guy was coming to town for a basketball game anyway.

Its a GM 12 bolt unit from a '68 Buick Skylark, non posi unit and not 100% sure on what gear ratio it is and I'm having a hard time finding the code on the axle tube (thats where it should be right?). The codes I'm finding on the axle tube are LE then a 11 U/K. On the pumpkin is 3917124N. Granted whatever it is I will more than likely gut it and swap out for all new gear and a posi.

Anyone have any info that would help me out?

Well I finally figured out why I was having troubles it was not the stock rear from that car, must have been swapped out at some point. Anyway the rear is from get this a '70 GTO measures 60 inches wide and has a gear ratio of 2.73 think that will have to get changed out...

great pickup on the rear even if you need to do gears and a posi - those are getting harder and harder to come by.

your TPI stuff is on the way in a few minutes, tracking will come via paypal email. fedex is on the way to town and i've got some errands to run.

Just remember not all GM 12 bolts are the same. The parts for a Chevy won't fit in a Pontiac rear for example, depending on the year. Also some non-Chevys had 12 bolt covers and 10 bolt ring gears. Might want to pop the cover and count to be sure.

Lord help me I'm actually thinking about keeping the 2.73 (at least for now).

Junkyard from what I have found this should be an actual Chevy unit, numbers on the pumpkin are exactly what you would find on a Chevelle. Though I know what you are talking about the differences. The first 12 bolt I ran across was one of the ones with 10 bolt guts. I will more than likely pop the cover tomorrow.

patgizz thanks for the update, look forward to getting that setup. I think the first incarnation of the car will be with the Quadrajet while I save up for the Megasquirt.

At that age it's a corporate rear end. Interchange won't be a problem.

They stopped the wonky axle stuff by the end of the 50's.

Good score.

Shawn

Got the complete rear for $150 off Craigslist, he originally was asking $250. Basically he found the car in a field bought it off the guy, sold the front end suspension bits to his buddy and was going to use this in his Chevelle. Then before he got it in he found a posi from one of his buddies so this went up for sale.

I'm thinking about switching to rear disks on it. Anyone know if there are parts I could salvage off some other rig that would fit the bill or do I need one of the conversion kits?

Just got home with the engine and transmission and 2 more wheels. Jeesh its been a busy weekend for the Cutlass.

Speedway motors has a nice, simple rear disc setup for those axles if you don't need parking brakes.

If you want to go junkyarding you can rob the backing plates, calipers and rotors off of a disk-braked S10 rear axle, this will give you parking brakes but they aren't that great since the park brake is a lever arm that actuates the caliper. Similar systems were used on RWD Cadillacs and F-body cars.

The best setup is one of the aftermaket setups that puts a drum style parking brake in the rotor.

Shawn

rebelgtp wrote: Lord help me I'm actually thinking about keeping the 2.73 (at least for now). Junkyard from what I have found this should be an actual Chevy unit, numbers on the pumpkin are exactly what you would find on a Chevelle. Though I know what you are talking about the differences. The first 12 bolt I ran across was one of the ones with 10 bolt guts. I will more than likely pop the cover tomorrow. patgizz thanks for the update, look forward to getting that setup. I think the first incarnation of the car will be with the Quadrajet while I save up for the Megasquirt.

i kept the 2.78 that came with my 3rd member. car is light and the 351 will have enough torque to make it go quickly

Yeah I had seen the ones on Speedway though around town here there are many places where parking without a parking brake is just asking for trouble. Especially up around my school on the hill.

I think there are a bunch of S10s in my local yard any ideas what years I should be looking at? Could the setups off the Cadi or F bodies be adapted or is the S10 the best bet?

Not sure what year of S-10 would work, I've heard of guys using it but that's all.

The F-body would be the 1979 - 1981 Trans-Am, not sure about the year for Cadillacs but I know it's late 70's sevilles.

You'll need the M/C and prop valve too.

Honestly though, disc brakes aren't really worth the trouble for the GM setups. I've done the swap a couple times using the F-body stuff and there isn't really much improvement over a well-set-up stock drum system.

Herb Adams made the drums work really well, you should consider what he did. Take a 1.25" hole saw and drill 5 holes in the drum face, between the stud holes. Keep the outside edge of the hole close to the braking surface but don't hit it.

Drill a few holes in the back plates where you can with weakening them as well. Now the brakes can breathe and they don't fade near as bad.

I've done this on three F-body rears now and I've autocrossed them with no trouble at all and saw a good improvement in braking.

If you really need a bit more oomph, the Buick GN guys use a S-10 wheel cylinder in the buicks to increase rear brake pressure. I'm not sure what the exact application is but I'm sure you could find it with the help of Google.

If you really feel that you need to spend time and money, I would use an aftermarket system. The factory systems just aren't worth the trouble.

Also. Three of my T/A's have the factory disc brake rears. Even when they're setup right with brand new parts, the park brake won't hold the car worth a damn. Put it in gear or put it in park every time.

Shawn

Thanks for the info Shawn I will do a bit of research and see what I can come up with. More than likely the drums will stay for awhile but if I do the supercharger setup I was thinking I want as much stopping power as I can get

Drums wont fail you over an autocross, they need several hard applications to begin to cause problems, I doubt you would get them hot enough, now if you are looking to road race I retract my statement.

Yeah I'm not sure I would ever try road racing this beasty though it is hopefully going to hit the autocross courses next year. Heck if I could figure out bringing it in under budget I just might use it as a challenge car .

Though I'm not sure how the value of the car would be considered? It was a free non runner, hadn't been advertised or anything was just told yep you can have it. Wish I would have drug the Monte Carlo home as well, that ended up getting crushed.

Hiw do i i.d. this rear end