[im4darush]

I'm preparing my 72 TA for street use, pulling the original HO and M22 so I won't trash them but I'm not sure what do do about the rearend. I can't find much on this subject, can the 10 bolt be made to handle an occasional thrashing with 450HP and street tires with a part time idiot at the wheel? I do have a 12 bolt but it's worth something? plus would need a gear ratio change and freshened up so that path would cost me an easy 3k and storage space for the original. Seems like building up the 10 bolt would be the easy path but can they be built to safely hold up?


Thanks for any advice!

[kingbuzzo]

I'm trashing my 10 bolt if that helps but not by choice...maybe she'll hold is my hope lol 😀

[PAUL K]

The posi unit is the week point followed by the axles. The 8.5 is stronger that most folks think. I ran hundreds of passes in the low eleven second range (550ish HP) before upgrading from stock axles. My brother started grenading the factory posi unit at the 10 flat mark (700ish HP).... I'd stick with the 8.5 but the 12 bolts definetly gets bragging rights.

[Mr Twister]

You will probably be fine as is, except for the fact your 53 year old unobtainable clutches are probably worn out. The common mod is to install a 30 spline carrier and 30 spline axles as an upgrade to the 28 you have now.

[JSchmitz]

8.5's will take a lot. Especially if it only sees street tires.

[unruhjonny]

Originally Posted by Mr Twister

You will probably be fine as is, except for the fact your 53 year old unobtainable clutches are probably worn out. ...

huh?

How about THIS?

or THIS?

Granted the second link says they are out of stock - but this should show that you can in fact find them...

[Mr Twister]

I looked 10 years ago and there were none to be found, also the first link is for Ford 8.8 clutches. I should be more clear. I was looking for GM parts not aftermarket.

[unruhjonny]

Originally Posted by Mr Twister

I … the first link is for Ford 8.8 clutches. …

Whoops, my bad - I did a quick google search for GM 8.5” clutches.

Originally Posted by Mr Twister

… I should be more clear. I was looking for GM parts not aftermarket.

oh - gotcha.

['ol Pinion head]

231127 was the original part number for the '71-87 8.5 posi clutches.. These were discoed by GM near 20 years ago & around $112 a pkg. Through the 90's, I normally bought em 8-10 boxes at a time @ $49- 60 a box. A Buick guy had the clutches reproduced in the mid 00's, they dont have the same swirl pattern, but they work fine. To properly shim up the side gear to spider clearance it takes having a good assortment of clutch shims. GM discoed the clutch shims for this style carrier even earlier, early 90's, so most guys didnt have a selection. One thing I've done is take a pair of used 8.5 clutches with the ears on them. Cut the ears off with the Dremel tool, then use these non splined clutches as the base of the stack to add a thin service or aftermarket clutch shim to. The OEM S spring carrier clutch shims will measure from .088 to .102 thick. The two of them are typically very close to each other in thickness. At the very least, you want to replace with the shims that came in the carrier. More often than not with original side gears & spiders, it will take being a .001 to .003 thicker on each side to tighten up the tolerance. The spider gears in these are larger than the small brittle spider gears that were used in original GM 12 bolt "chevy" posi carriers. Just one of the advantages over an original Chevy 12 bolt. With original 12 bolts the vast majority of them do not have much cast steel under the cap mating area, this is a weak area. So are the thin stock 12 bolt caps.

[OCMDGTO]

$3k should get you a ready to run 12 bolt/9"/Dana 60. Why mess with old junk unless you're going for matching #'s?

[78w72]

10 bolt will be fine with street tires if its in OK condition as far as axles or the center section.... if the clutches are worn out thats usually noticeable, if it still spins both tires on a burnout/launch they are probably OK. Change the fluid if its original/old & add a bottle of GM limited slip additive.

if you plan to do drag racing with a drag radial or slicks, thats when you want to beef up the rear end, with good parts like a eaton or other brand center, 30 spline or even 28 spline but forged street axles it should handle 450hp fine. i have a 10 bolt with the same parts i mentioned plus a stud girdle cover at 500+hp & lots of low 11 runs... i worry more about the driveshaft than the rear end.

[unruhjonny]

I want to piggy back on what has been said;

It is my belief that as originally built, GM intended either the tires to spin, or the clutch to slip;
In people quest for better e/t's the clutches are typically upgraded to have far more holding power than OE, and likewise drag radials have alot more bite than OE tires;
It is my opinion that when you upgrade both these parts, all you are doing is exposing the weakest link - either something is going to spin/slip, or something is going to break.

If you stick to street tires, and an OE replacement clutch, you will probably never have issues with a 8.5" differential.

[65madgoat]

450hp and torque you will be more than fine.

[im4darush]

How awesome it is to receive the knowledge / kindness of fellow Pontiac men!


All good posts, I'm now able to get off the fence. OCMDGTO's post was an option that had not come to mind but seems to be a good one which keeps the original completely safe, all new, no worries about finding parts, work not being done right at a local shop, better than the stock 12 bolt which when sold hopefully will cover a good chunk of the cost for a new one. Got a plan and am thankful for the help.


Guess it time to play the scratch off lottery to find out what this 12 bolt may have come out of as I can't remember from 35 years back. Then the big question, what's it worth?

Originally Posted by OCMDGTO

$3k should get you a ready to run 12 bolt/9"/Dana 60. Why mess with old junk unless you're going for matching #'s?

[Formulabruce]

Things to consider.
1. The 30 spline carrier and axles would be my GO TO upgrade for a 10 bolt. Pinion is already same as a 12 bolt....
2. Do NOT, yeah Do NOT .. forget about the U joints, which QUICKLY become the weakest Link. My HO tears stock ones like paper.
3. Rebuilding a Safe - T- Track Carrier ( posi) .. Clutch packs now available and whole carriers have Carbon Fiber clutches, and NOT steels.
You can burn them up quicker. they are Made to slip.. and Not for drag/ street racing.

[Scarebird]

...or you can go the helical gear route:

https://www.summitracing.com/parts/etn-912a556

[fbody_mike]

I've never destroyed an 8.5 10 bolt with my right foot, but this was always in the 300 to 400 hp range and street tires. I have killed one once when the cross shaft retaining bolt backed out and let the shaft contact the housing at 60 mph.

How come no one here has mentioned the gear ratio yet? Seem the higher (numerically) the ratio the weaker it will be, just like a manual trans, the deeper the first gear, the easier it is to grenade it.

[im4darush]

So yes gear ratios should be known, thanks FBODY Mike



That plan is for a 3.08 rear gear with a new super (Z)M22 (link below) which has a 2.96 first gear and 1 to 1 4th gear. The equally spaced 1st thru 4th sounds to be better than the Tremics. Only four gears and no modding the floor pan.



https://midwestmuncie.com/product/au...ear-end-gears/

[Formulajones]

Anyone here old enough to remember Dave Henninger. He was really the first that I recall showing everyone how durable a beefed 8.5 can be back in the mid 1980's.
He was running 8's at 160 mph.

I'm building one for a 500hp application with the true trac and 30 split e axles etc .. and not even worried about it.

['ol Pinion head]

Would strongly bet I've built up more 8.5 A-body, F-body, & X-body rears for performance use than anyone on this board. That noted, there are numerous advantages of the 8.5 10 bolt build over the stock 12 bolt build. Still you get the "I GOTTA HAVE A 12 BOLT". Have no problem filling that order if I have the right housing, but for the most part, the 12 bolt "need" is nothing but misconception. The early 8.2 10 bolts gave 10 bolts a bad rep, not the 8.5's.

On any leaf spring 8.5 or OEM 12 bolt build if the car is anything more than a putt-a-round cruiser, it's smart to have the bare clean housing put in a line up bar, & then have the center housing welded to the axle tubes. I prefer having this weld performed with a TiG welder, end up with a very clean result. I don't own a nice TIG setup yet, but have installed my lineup bar in multiple dozens of c-clip axle housings & hauled them to buddys that can flat lay down nice welds. At near 500 hp & 575 lbs of torque, I would have no concern building up an 8.5 F-body rear for such a manual trans street/strip build. Built up 8.5's for many cars that were subjected to much more & for many passes on slicks.

To swap out for PureStock races, I built up an 8.5 A-body (bolt in axle) rear with an early Eaton 30 spline posi & a pair of custom Mosers (purchased back when they were still in the mid $400 range). Set the rear up with a set of old stock US Torqueline 3.73's. The center cover area on that hsg I had drilled & tapped to accept a 12 bolt cover. Only for appearance sake, as the rear will get swapped in & out of my HO T-37 Coupe & eventually into my HO GT-37. Both the gone through original 3.31 & 3.55 12 bolt posi rears out of my 71's are being detailed for national show use only! Having built so many of of each style rear, am much more impressed with the strength of the built up 8.5's than similar built up stock housing 12 bolts. Could have easily built up a 3.73 12 bolt posi rear off another housing in the racks. If I had gone that route, would have used a brute strength carrier have on the shelf & a thin ring gear 12 bolt ring & pinion set. Have done that exact setup before. Once was behind a little over 500 hp built 462 HO, very similar weight A-body as my 71's. That particular '71 HO A-body with thin 3.73's, brute strength carrier & pair of Mosers regularly ran on slicks, 11.40's with a best into the low 11.30's. Have built same ratio 8.5 A-body rears that footbraked ran well into the mid/low 11's, then sprayed into the mid to high 10's.

Absolutely amuses me that for the 12-13 sec street car builds, the recommendations are "buy a new Moser 12 bolt or Dana60". Price out those musclecar-pak deals, they ain't cheap. With sales tax & delivery, knocking on 5K a rear. Now, if I owned a '69 A12 Superbee it would have a Dana 60 in it.