[b-man]

Originally Posted by Skip Fix

I would say if you can get good price on a 8.5 with bolt in axles up grade at least to good 28 splines or jump to a 30 spline carrier and axles and it will be as good as a 12 bolt for what you are doing.

This.

And this, deep into the 10s with a 30 spline Eaton and Dutchman axles:

https://forums.maxperformanceinc.com...14&postcount=7

[Jay S]

Originally Posted by vicgto

How do you distinguish the quoted Cutlass 8.5 from a Chevy 8.2, do you have a rear cover or bolt to bolt dimension or some other distinguishing feature?

It does not have the lower corner extrusions on the bottom of the case like the 71 and 72 Buick and Chevy 8.5.

The Olds 8.5 cover is unique. The protrusion on the cover is just on one side, from 12:00 to 3:00.

[Jay S]

When looking from the back the other 8.5s have the triangle shaped extrusions that stick out on each side on the bottom of the case right in font of the cover.

[Scarebird]

['ol Pinion head]

Originally Posted by Jay S

When looking from the back the other 8.5s have the triangle shaped extrusions that stick out on each side on the bottom of the case right in font of the cover.

There are actually two different '71-72 versions of the Round Cover Olds 8.5 A-body housing. the more common A9 housing end style, & the larger A10 axle bearing style with its special backing plates.

What's nice, is the round cover used on c-clip housing 8.5 housings is a direct bolt on to the Olds, so I always swapped on the round cover for outright sales, ESP total builds to present at the big swappers.
Early on in the early 90's, on many an 8.5 A-body build I also carefully ground down the big raised A's & O's off the different center housings. Also ground off many a casting number. The reason was to throw off wannabe future competitors, this was when i was selling fresh built up 3.08- 4.10 ratio 8.5 posi rears. It took longtime early 70's diehard Olds 442 or Buick guys to have a clue of what the "Big10" rears were. At the time, so many of the Olds & Buick guys didn't pay attention, and the Checy guys were clueless. They were also of the drift that their car needed a 12 bolt posi, so they totally ignored all the inherent weaknesses of a stock 12 bolt posi "Chevelle" rear. I used to call it the "I gotta have a 12 bolt" syndrome, LOL.

[Scarebird]

OPH: does GM still use the 12 bolt in any production capacity?

I know my '16 Sierra half ton has a 8.5, not sure if new trucks still use them...

[i82much]

i think there used to be a different 12 bolt for trucks .... is your truck 8.5 the same as the passenger car 8.5?

['ol Pinion head]

Originally Posted by Scarebird

OPH: does GM still use the 12 bolt in any production capacity?

I know my '16 Sierra half ton has a 8.5, not sure if new trucks still use them...

On any form of 8 7/8" GM 12 bolt, not, that I've ran across. The Chevy truck 12 bolt ran its course after believe it was the 83 models. On the latest 8.6 GM 10 bolts in pickups I have not kept up with their changes & am not doing bench builds on them. Over the years the local GM 1/2 ton 4WD beater playing in the mud crowd really wore out my patience.

12 bolt truck rears... the 76-79 Chevy Big 10 & Heavy Half Pickups used the 12 bolt truck rear, as well as stronger springs front & rear. My Dad special ordered new a '78 short wide Big 10 Silverado, easily the best GM pickup that ever went through our family. At one time Moroso had a contract with Eaton & offered low ratio Brute Strength HD Eaton posi carriers for the 12 bolt truck rears. Set up several of the Brute Strength truck carriers & new gears in core housings in place of destroyed housings that Eaton Govlocks had taken out the ring & pinions & lh cap mating area. Am not that big a fan of the 12 bolt truck housing & gear design for 4000 lb street/strip 1/2 tons. If I was presented with building another such rear, would most likely go a different direction.

[Skip Fix]

I think my 2015 Silverado has some 9+ inch rear in it. Has towing package if that makes a difference. My 97 has an 8.5 10 bolt POS Guv lok 30 spline

[i82much]

Originally Posted by Skip Fix

I think my 2015 Silverado has some 9+ inch rear in it. Has towing package if that makes a difference. My 97 has an 8.5 10 bolt POS Guv lok 30 spline

i think they use the 9.76 in the NHT trucks.

[Skip Fix]

I think you are right about the size -rear cover style but bigger than the old 8.5.

[Jay S]

There are combos running chevy 8.2s with 31 spline spools with stronger aftermarket gear sets for class racing. Apparently the combo of aftermarket gears and the cover brace must help them hold up to the abuse to some degree.


After 1999 the 8.5 became an 8.6 (8.625” ring gear, and all had 30 spline axles). The gears interchange, the 8.6 carrier will drop in a 8.5 with different carrier bearings, but it doesn’t have a reputation be being very strong in OEM versions. The 8.6 ring gear and pinion are more expensive.

Here are some torque ratings for forged axles if your curious where your combo falls. Per axle

(Engine TQ)X(first gear ratio)X(axle ratio)= Maximum TQ to axles for street tires

28 spline = 4800
30 spine = 6200
31 spline = 7000
33 spline = 8200
35 spline = 9600
40 spline = 12000

Safety factory of 200%, since it is per axle.


The numbers get interesting when you consider drag racing with slicks and dead hooking. The torque converter multiplies the engine tq for a small instant. After the car launches, it can only multiply the engines actual torque, the tq converter is no longer much of a factor. For drag racing with sticky tires multiply the axle torque numbers with additional multiplication of 2.5 from the torque converter. Not hard to see why we break stuff drag racing. The carrier must carry those axle torques also.

[i82much]

I feel like a lot of these decisions you make when building a car are really strongly influenced by what you might already have, what used stuff you can find locally, etc. I mean if i have an 8.5 or 12 bolt in my shed or I find a decent price on craigslist, I'm not dropping 3 grand on a brand new 9 inch or dana 60 if am building a 500 hp car. But if I'm going to buy a brand new rear assembly anyway, I have a hard time seeing why I'd go with anything other than a 9 or a 60.

[Skip Fix]

Yea I have a '66 Chevelle 12 bolt I'm torn between throwing in the 65 GTO project or throwing a posi I have in it and selling it and buying either a new 12 bolt or 60-stick car.

['ol Pinion head]

Originally Posted by i82much

I feel like a lot of these decisions you make when building a car are really strongly influenced by what you might already have, what used stuff you can find locally, etc. I mean if i have an 8.5 or 12 bolt in my shed or I find a decent price on craigslist, I'm not dropping 3 grand on a brand new 9 inch or dana 60 if am building a 500 hp car. But if I'm going to buy a brand new rear assembly anyway, I have a hard time seeing why I'd go with anything other than a 9 or a 60.

Have three '71 & 72 A-body's that are factory 455 HO cars. All have their McKinnon 12 bolt posi's in them. The lightest of the 3 was raced near 50 years ago with a General Kinetics cam, headers, & slicks eventualy into the very high 11's. With very mildly cleaned up heads, it even ran a little quicker. That car ended up breaking a stock axle & did some damage to one 1/4, then it was parked. Since then i have rebuilt the original 12 bolt XVK but when all the bodywork is finally complete it will only be installed for show duty.

On my T-37 Coupe, it's an original M22 car, but I've shelved it's M22 & XUK coded 12 bolt as they a weak point. The substitution is a not that budget friendly M23W backed by a 30 spline Moser axle TruTrac built up HD 8.5 A-body rear. The goal is to get the Coupe down consistently into the mid 12s in PureStock trim . There is a recipe to accomplish that, & I could easily build an upgraded 12 bolt posi up to swap back & forth. Instead am building another 8.5 A-body.. With the parts have on hand, it will mimic one I built earlier & currently is in a low 11 sec GS clone. That was not a $3300 or $3500 expense, right under 2K at the time. Quality parts have gone up some, & so has my labor, more like $2400 today.


Just in blueprinted 28 spline stock axle form, the same 8.5 A-body rear has proved to be very comfortable in a foot braked auto A-body in the high 11 second range, have built way too many that lived there for many many years. In the early years, nearly all were assembled with nothing but correctly rebuilt 8.5 S spring carriers, & used 3.42 or 3.73 GM gears. As demand for other ratios took off, & builds with much high torque engines were presented to me, I built with more & more with aftermarket posi carriers & axles & US Gear Torque-line gearsets. It was very disappointing to see US Gear get bought & then their excellent ring & pinion line discontinued.

For axle retention, am a big fan of bolt-in tapered bearing axles, & prefer that method of axle bearing many times over aftermarket axle c-clip axle retention or Moser's new 12bolts with ball bearing axle bearings. For EXTREME usage, running different length tracks in what amounts to a much quicker bracket car (1.40's & quicker 60 ft times) have no problem investing in a properly built beefed up aftermarket 9" Ford rear. In a stock appearing very slick high 10 to 13 sec Pontiac, am not impressed at all in looking up under the rear bumper & seeing a 9" or a 9" & one of those goofy curved aftermarket swaybars. Have about the same feeling when run across a similar slick very collectible Pontiac & it has a 6.0 swapped in it

[AG]

My car uses a 1972 nova 8.5" rear with tubes welded to center section, Strange spool, Moser 33 spline axles, Ford ends with bolted in axles, Strange billet bearing caps, chromoly pinion yoke and an LPW cover. Car does a 1.45 60' time and goes 10.0x, haven't had any issues with it. Would I rather have a 9", of course.

[Cammer-6]

OPH may be able to answer this...
Old racer thats very well known here,told me he was running 8.5 guts in his 8.2.
He said NHRA balked but couldnt find anything in rules to stop it.

[Verdoro 68]

Installed a T/A cover as a stop gap on my way to a better rear end. I didn't realize how beefy these things are. If I can find an 8.5 I'll just transfer it over. The oil looked pretty clean for being 10 years old.

[b-man]

Well now it’s settled.

You need to get the 8.5 since it matches your new TA Performance cover.

The 8.5 is more than plenty strong enough for what you’re doing with your car.

[Verdoro 68]

Originally Posted by b-man

Well now it’s settled.

You need to get the 8.5 since it matches your new TA Performance cover.

The 8.5 is more than plenty strong enough for what you’re doing with your car.

I think so too. I'm going to be patient and see if I can find an 8.5 core locally to build up. Maybe hit up a few swap meets in the Spring and see what's out there.

Anyone know the difference in driveshaft length between an 8.5 and an 8.2?