[pittbull7934]

My friend told me that to tell if a rear end is a 12 bolt is too look at the ring gear, and the ring gear should have 12 bolts on it.

He said to look at the outside of the pumpkin and count the bolts doesn't do anything.

He said that GM made rear ends that had 12 bolts on the pumpkin but were esentially 10 bolts on the inside.

Is this true? if so, is there a group of casting numbers i can look for? I am trying to find a 12 bolt for my el camino, and am now worried about having to take every 12 bolt i look at apart.

I know I am new to this site and have been posting many threads, but I would like to say thanks to everyone for your patience and help!

[joe_padavano]

Originally Posted by pittbull7934

My friend told me that to tell if a rear end is a 12 bolt is too look at the ring gear, and the ring gear should have 12 bolts on it.

He said to look at the outside of the pumpkin and count the bolts doesn't do anything.

He said that GM made rear ends that had 12 bolts on the pumpkin but were esentially 10 bolts on the inside.

Is this true?

Your friend knows what he's talking about. Contrary to popular belief, the number of bolts on the cover do NOT indicate the strength of the axle. It is the ring gear that matters. GM made a lot of oddball axles over the years and there is a lot of confusion.

The Chevy 12 bolt has twelve bolts holding an 8.875" ring gear to the carrier. It also happens to have 12 bolts on the rear cover. The Oldsmobile Type O axle also has 12 bolts on the cover, but the ring gear is only 8.5" and is only held on with ten bolts. This axle is no stronger than the "corporate" ten bolt 8.5" axle, but many vendors try to sell this axle as if it were a real Chevy 12 bolt. The Type O does have the advantage of having the axles retained at the outboard ends - there are no C-clips to worry about. These are the "Olds axle ends" that people add to their Chevy axles.

Similarly, there were Pontiac axles used under full size GM cars that only have ten bolts on the cover but use 12 bolts on the 8.875" ring gear. There were even some Olds full size axles with 9.3" ring gears. The problem with these oddball axles is parts availability, however.

There are some indications from the shape of the rear cover and so forth, but the only sure way to tell is to pull the rear cover and look.

[cobalt327]

Also Chevy truck "12-bolt" diffs are not as strong as a "real" 12-bolt car diff.

The pinion of the truck diff isn't even as big as a "corporate" 8.5" 10-bolt. (1.438" for the 12-bolt truck, 1.625" for the corp. 10-bolt and 12-bolt car diffs). The truck ring gear is the same diameter as the car 12-bolt.

[pittbull7934]

So, the best way to tell if it is a real 12 bolt is to count the bolts on the ring gear?


Thanks for your help!

[C-10]

Originally Posted by cobalt327

Also Chevy truck "12-bolt" diffs are not as strong as a "real" 12-bolt car diff.

The pinion of the truck diff isn't even as big as a "corporate" 8.5" 10-bolt. (1.438" for the 12-bolt truck, 1.625" for the corp. 10-bolt and 12-bolt car diffs). The truck ring gear is the same diameter as the car 12-bolt.

Your info is slightly misleading. Design wise these two 12 bolt rears are the same, which in itself, is the weakness. Pinion shafts are hard, very tough stuff. The output of a trans would snap way before a 1.438" pinion shaft. Not to mention axles.

[ericnova72]

Originally Posted by C-10

Your info is slightly misleading. Design wise these two 12 bolt rears are the same, which in itself, is the weakness. Pinion shafts are hard, very tough stuff. The output of a trans would snap way before a 1.438" pinion shaft. Not to mention axles.

Not really misleading. The 1.438" shaft has less meat at the end the pinion gear is on. What happens is the pinion gear is squeezed away from the ring gear ("deflection") under large power loads and tears up the gear set.

Your right about outright snapping the shaft off compared to axles or trans output shafts, but that isn't what happens. The teeth get tore up and broken off.

The 12-bolt truck is inferior to the 12-bolt car, and the aftermarket availability of narrowed truck rears for race use shows this. No one offers a 12-bolt truck rear package despite the abundance of them in wrecking yards.

[pittbull7934]

I found a rebuilt 12 bolt out of a 66 chevelle, that should just swap right out with my 68 elky?

also, do you guys have any suuestions for gearing?

I want to run between 2500-3000 rpms at 75/80mph.

it looks like i will be installing a built turbo 350.

I have access to 3 powerglides.

My 383 should be pushing over 450hp/ft lbs

Could you guys offer some suggestions for rearend gearing and what tranny to run?

I want to have a driver but also something i can take to the track once or twice a year.

[junior stocker]

Originally Posted by joe_padavano

Your friend knows what he's talking about. Contrary to popular belief, the number of bolts on the cover do NOT indicate the strength of the axle. It is the ring gear that matters. GM made a lot of oddball axles over the years and there is a lot of confusion.

The Chevy 12 bolt has twelve bolts holding an 8.875" ring gear to the carrier. It also happens to have 12 bolts on the rear cover. The Oldsmobile Type O axle also has 12 bolts on the cover, but the ring gear is only 8.5" and is only held on with ten bolts. This axle is no stronger than the "corporate" ten bolt 8.5" axle, but many vendors try to sell this axle as if it were a real Chevy 12 bolt. The Type O does have the advantage of having the axles retained at the outboard ends - there are no C-clips to worry about. These are the "Olds axle ends" that people add to their Chevy axles.

Similarly, there were Pontiac axles used under full size GM cars that only have ten bolts on the cover but use 12 bolts on the 8.875" ring gear. There were even some Olds full size axles with 9.3" ring gears. The problem with these oddball axles is parts availability, however.

There are some indications from the shape of the rear cover and so forth, but the only sure way to tell is to pull the rear cover and look.

Were'nt the Pontiac-Olds axles 8.2 ring gear, NOT 8.5? (Just to add to the confusion)

[WDCreech]

Originally Posted by junior stocker

Were'nt the Pontiac-Olds axles 8.2 ring gear, NOT 8.5? (Just to add to the confusion)

Until the '69 model year, yes they were.

[Bryan59EC]

Originally Posted by pittbull7934

I found a rebuilt 12 bolt out of a 66 chevelle, that should just swap right out with my 68 elky?

I want to have a driver but also something i can take to the track once or twice a year.

You must note that the 64-67 rear ends are narrower than the 68-72 rears.
The will physically interchange, but you must pay attention to the size of the tires and wheels you will be using.

You could just send me that 66 rear
cause it should bolt up nicely in my 66 Elky, and once the 307 now in the car fails, I have a fresh 454 in the garage

[cobalt327]

A 3.08 will be 2772 RPM @ 75 MPH w/28" tires. Add TC slippage to this.

A 3.42 will get you 3078 RPM, all else as above.

A 3.73 will be 3357 RPM, as above.

Other ratios are available in between these, but these are readily available from all the manufacturers.

I had an OEM 3.23 gear set for my Camaro's 12-bolt and liked it a lot. Had a number of engines in the car throughout the time I owned it, but the bigger engines liked it, too.

For a SBC, I'd lean more towards at least the 3.42. Better yet, IMO, would be a 3.55-up ratio in that 'Camino

[joe_padavano]

Originally Posted by junior stocker

Were'nt the Pontiac-Olds axles 8.2 ring gear, NOT 8.5? (Just to add to the confusion)

The Type O was built from 1967-1970 and only used under Cutlass cars. It is a different axle from the 8.2". The Type O ring gear is 8.5".

[OLNOLAN]

Hey Folks, I figured that I'd add to the confusion regarding 12 bolt rears. The inner pinion bearing issue is partially correct when people say that the truck 12bolt is not as strong as the car 12bolt. BUT, it appears that only two rearend housings are being addressed, the 1.438" bore/3.00" cup housing and the 1.625" bore/3.25" cup housing. There is another 12bolt housing that uses a larger 1.875" bore/3.50" cup. The catalog listings for this bearing setup are 8.875 ring gear 73-75 chevelle wagon, 76 chevelle, 73-76 full size chevy wagons,71 full size chevy w/P axle and 77-81 1/2 ton chevy trucks w/posi. This big bearing rear appears to be the one you want. So the general statement that 12 bolt car rearends are stronger than 12 bolt truck rears is not always correct, it just depends on which housing you have. Now, I need a rear stamp decoder for Chevy truck rears 73-82. Who has the manual with all the codes listed to make junkyard spotting the posi rears easier. I've posted a couple of times trying to find this info but have not had any luck. If anyone could post the codes for the truck rears it would be greatly appreciated. Later, OLNOLAN

[pittbull7934]

Originally Posted by OLNOLAN

Hey Folks, I figured that I'd add to the confusion regarding 12 bolt rears. The inner pinion bearing issue is partially correct when people say that the truck 12bolt is not as strong as the car 12bolt. BUT, it appears that only two rearend housings are being addressed, the 1.438" bore/3.00" cup housing and the 1.625" bore/3.25" cup housing. There is another 12bolt housing that uses a larger 1.875" bore/3.50" cup. The catalog listings for this bearing setup are 8.875 ring gear 73-75 chevelle wagon, 76 chevelle, 73-76 full size chevy wagons,71 full size chevy w/P axle and 77-81 1/2 ton chevy trucks w/posi. This big bearing rear appears to be the one you want. So the general statement that 12 bolt car rearends are stronger than 12 bolt truck rears is not always correct, it just depends on which housing you have. Now, I need a rear stamp decoder for Chevy truck rears 73-82. Who has the manual with all the codes listed to make junkyard spotting the posi rears easier. I've posted a couple of times trying to find this info but have not had any luck. If anyone could post the codes for the truck rears it would be greatly appreciated. Later, OLNOLAN

there are great casting number books on amazon, have you tried there?

[OLNOLAN]

Originally Posted by pittbull7934

there are great casting number books on amazon, have you tried there?

Hey Friend, I've found some pretty good sources for casting #s on the net,free. But what I'm looking for is the axle build stamping #s. It's the stamped ID on the right hand axle tube that indicates what went into the housing when it was built e.g. posi,no posi,gear ratio,date built,etc.I have found some decoders for cars but not the trucks. I think a book something like Chevrolet By The Numbers, GM Truck Manual,etc. may have what I need but I don't want to buy the whole manual just to get the stamping decoder. Later, OLNOLAN