DarrylJ

I have a 12 bolt 3.23 gear rear in my 70 Supreme conv. It is a chevy rear from a 69 Chevelle. The rear has made some clanking noises once in awhile, especially if i gas it a little in reverse. Now the car is a one wheel peeler as they say. Any thoughts as I have never had one of these apart?

J-(Chicago)

Get a Moroso Brute, and junk the Auburn.
http://store.summitracing.com/egnsea...=KeywordSearch

If you want a 3 series, get an eaton.

Just my opinion.

monzaz

Well first of all the 3.23 does not make sense in the Chevy 12 bolt??? That is a 12 bolt OLDS gear ratio NOT Chevy 12 bolt. Chevys ratio was 3.31 or 3.55 3.08 3.73 3.90 4.10 etc.

12 bolt olds had ratios such as 2.78, 3.08, 3.23, 3.42, 3.91, 4.33

Noise... Is this a posi or open rear diff? If open single wheel peel it could be spider gears worn...ring and pinion worn and or chipped tooth. Lastly very well could be one of many bearings in the rear from pinion inner or outer or carrier or even axle bearing. Or a loose pinion nut letting the pinion go in and out of the housing to make a clunky sound. Jim

joe_padavano

It's very easy to tell the two axles apart. The Type O (Olds) axle has 12 bolts holding the cover, which is smooth. The Chevy axle has 12 bolts holding a cover with a raised inverted "V" stamping on it. More importantly, the Type O is NOT a "12 bolt" axle. It uses only ten bolts to retain an 8.5" ring gear. The true Chevy 12 bolt uses 12 bolts to retain an 8.875" ring gear.

Allan R

Joe,
Got me confused by the O being not a 12 bolt axle thing. How is the Olds 12 bolt axle labelled? I know that they came with w27 diff covers and a true w27 wasn't just the cover, it included the diff housing too, right? My 72 uses a 10 bolt and 8.5 ring, so I'm jiggy with that. What code would be used with a 12 bolt Olds axle (posi or non)? Seen lots of early Olds with what is claimed to be an O axle/diff.

joe_padavano

This is a common misconception. Most people think "10 bolt" or "12 bolt" refers to the number of bolts on the cover. It does not. It really refers to the number of bolts holding the ring gear to the carrier. On Chevy axles, the number of cover bolts conveniently corresponds to the number of ring gear bolts. Unwitting (or unscrupulous) sellers frequently advertise the Type O axles as "12 bolt" axles. I suspect the buyers often get a rude awakening.

The Type O was used exclusively in Cutlii from mid-1967 through the end of the 1970 model year. It has an 8.5" ring gear with TEN bolts and bolt-in axles (no C-clips). Parts are unique to this axle. As a trivia point, Cutlii built in Canada during that period received true Chevy 12 bolt axles, not Type O axles.

The W-27 option included both the aluminum cover AND an aluminum center section in the axle housing. The same steel axle tubes were pressed into this aluminum housing and a rosette weld held them in place. The cover only was offered over-the-counter by the dealer parts counters, in both Type O (for 1970) and 10 bolt (for 71-72) versions. Aftermarket vendors also sell the W-27 cover drilled to the Chevy 12 bolt pattern.

Allan R

Joe:
You nailed it! I did think the 10 and 12 referred to the cover bolts. I bet there are more out there who thought this too.

I had heard that the Olds O axle was only till 1970.

Question: since I have a 8.5 diff housing, how difficult would it be to change to a posi carrier and 3:42 gears? Hoping it would be an easy install for an Eaton or Auburn. I don't know how many splines my axles have without taking them out and counting. Do you know a quick way to find out based on the axle that is currently in? (I suspect it is 30 splines) It was produced in May of 72 in Lansing.

monzaz

tHE oLDS USED THE 8.5 in 1971 1972 along with the Buicks. The Pontiac used the 8.2 10 bolt righ tto the end of the muscle car A-body era 1964-1972.

You will have NO problem finding parts for your 8.5 Olds 10 bolt at all.
-Rear is 28 spline Bolt in axles 45 degree cut LIKE chevy axles. 1970 and older olds and buick axle 28 spline were 60 degree cut. Different
-you can use a c-clip type or bolt in type posi carrier for your cutlas.
-Eaton or Auburn and even true trac will bolt right into that housing 10 bolt 8.5 GM.

We usually have Moser make up 30 spline axles for these rears and we use the 30 spline 8.5 units aftermarket of GM from 1990 -1996 and this reallyu makes a very nice strong 8.5 rear for racing or street.

Good luck and have fun with the cars. Jim

88 coupe

That qualifies as a gross understatement.

Norm

geckonz08

[quote=joe_padavano;46928]

Cutlii quote]


Love the plural

monzaz

Did you ever figure out which '12 bolt' rear you have?

O type oldsmobile or 12 bolt chevy.

You said it was posi and it is not posi anymore correct? Clutch pack may be totally worn out of spec...or you could have shattered a clutch and it broke into piece leaving the clutch pack one shy and cause the whole preload to be gone making the rear a open differential . Hope that helps...either way you have to crack that rear open. Jim