Torque/HP Limit of an 8.5
 

A fella near me will sell me an 8.5 Posi rear out of a 72 Nova for $250. Currently it has 3.73 gears. I though it may be a good core to build for my bird, assuming it is worth building. How much should I expect to spend getting one built - all new components including gears, bearings, axles, c-clip eliminators, etc? And, once built, what is it's power limit with good traction?

You can go with aftermarket 30 spline posi & axles & build one that will be quite strong. I've read that the pinion is aprox the same diameter as a 12-bolt pinion.

Price will probably be WELL over $1000. Maybe closer to $2000, including labor.

I think 'ol pinion head has built quite a few. So maybe he'll chime in.

I had my wife's 8.2" 10 bolt rebuilt this past winter. Minor upgrades only, but went with a yukon posi carrier, 3.36 gears, new axles and an install kit. If I recall this ended up being around $650.00 total shipped from Quick Performance. I had a local shop do the install as I've never setup a rear-end before. Around 650ish for the install.

I would assume c clip eliminator kit is going to add to that expense, but I wouldn't see why an 8.5 10 bolt with a decent set of 31 spline axles wouldn't support around 400 hp at the tires.

8.5's are tough as nails. There are a couple of sore spots with them however which I found very quickly when I started hooking up hard and hitting mine with big power.

The factory posi carriers have very hard spider/side gears in them. I blew two of them to bits in a dozen runs. Switched to an Eaton and fine now for nearly 20 years. The Eaton has more spring tension for the posi unit vs the factory "S" type, and the spider and side gears have greater contact area and not nearly as brittle as the factory parts.

The axle tubes need to be welded to the center section as they have small factory welds that can let loose.

Mozier axles and c-clip elim's is a good idea, might as well upgrade to the larger axles and get the bigger carrier at the same time, IMHO.

Never had any issues with ring/pinions, I use and prefer US Gear. Richmond is good, but can be just a tad noisier but I don't hear good enough these days to even matter.

1350 yokes are readily available as well, another place to upgrade.

The leaf spring perch welds need to be checked and reinforced on that unit. I ripped mine loose after 200 or so runs with good traction. Nothing else comes to mind at the moment and mine has been flawless now nearly 10 years since the upgrades were done......Cliff

They're very tuff properly prepared.

We found their limit to be around 10 flat in a 3700 lbs. car. The posi unit is the weak point after a basic build that has already been mentioned. A Tru-trac might be your best choice if you want a posi and want to keep going faster or a spool will be good for deep into the nines.

Years ago when starting on my TA I talked to Tom at Tom's Differential's(RIP) about axles. He said the stock posi will break first. The Eaton posi comes in 28 or 30 spline and 30 spline axles for a 10 bolt-thinner C clip "button" than 12 bolt axles. In my 3750lbs TA (when I had the light wheels on it) ran high 10s an I have an Eaton 28 spline and factory axles! MPH shows 550-575 HP depending on whose calculator. It is getting 30 spline axles and spider gears when I freshen it this winter. Factory yoke uses a 3R U joint and it is almost as big as a 1350.

Seen some in the 9s with spools as Paul said even with a heavy spray hit.

I ran into the high 10's at 123mph with a stock rear..I've upgraded to a Eaton and 30spline axles.
X2 on Cliff's response..do it all..plus it's pretty cool having the "stock" housing doing the work..driveshaft upgrades are relatively cheap also..
For most here the 8.5 is plenty strong

Thanks for the responses. I can get a Moser 12 bolt built w/30 spline axles, choice of ratio, monster caps and bolts for carrier, Yukon dura grip or Eaton posi, 1350 yoke for $2100 plus freight. If the 8.5 can get done for significantly less I’d consider it.

I have one in my race car. Tubes welded to center, Ford ends, Moser 33 spline alloy axles, Strange spool, Strange billet bearing caps, LPW cover running numerous 10.0x runs in a 3500# car and no spline twisting. No trans brake.

You'd have to add up the cost of setting one up. Way back when I did mine the savings wasn't that much over just buying one from Mosier (12 bolt) already done.......Cliff

I recently bought a Moser M60, looks brand new 3.73 gears with a clutch type posi. Found it on Craigslist for 1500. Keep your eye out.

Dave Henninger showed everyone in the late 80's early 90's that the 8.5 was strong enough to hold up fine in his 8 second 2nd gen Camaro once it was properly prepared. Believe he eventually got that car down in the 8.50 range at well over 160 mph.


I'd say $250 for a 72 Nova 8.5 with a positraction and 3.73's is a decent buy as long as everything looks good inside. You could probably run that as is and be just fine and upgrade things later as time and money allow. With street tires you'll never hurt it. Stickies and good traction will test the stock posi. Can always upgrade the posi later with a 30 spline setup and swap in a new set of stronger axles at that time.

They have the same 1.625 pinion diameter as a 12 bolt. The only real difference is a slightly smaller diameter ring gear and 28 spline axles. GM later went to 30 spline in these things in the late 80's and up truck applications.

Skip, did you tell me to use the Strange brand "C" clip eliminator bearings, for a cars that are street driven? Tapered bearings in theirs?

Yes Strange has a C clip eliminator that uses taper bearings and dual seals. Looks like the normal 3.150 taper bearings with a machined sleeve to go n the axle.

But if you go weld on ends you can get the 3.150 taper bearing with new axles that don't need a spacer. I have those in in 12 bolt int he Camaro. Did have to open backing plate up to fit them in.

My current 8.5 rear is from a 2nd gen TA. Anyone know off hand the width on this and how it compares to the 72 Nova rear?

I raced mine for a couple of years with C clip elims on stock axles w/o issue. I decided to upgrade to Mosier axles around 2003 and never looked back. I'm using the stock diameter 28 spline axles, with many hundreds of runs in the 1.59-1.64 range and not a grumble from anything yet.

I did find some play in Brute Force U-Joint last time I had it on the lift, nothing alarming but have a replacement ready to go in.

I compared them many years ago and the T/A's is wider but not by a lot, nearly as I can remember.........Cliff

" The axle tubes need to be welded to the center section as they have small factory welds that can let loose..."


If you plan to use sticky tires, this is an absolute MUST.

Because I ran out of time, I tried to get by 1 race with only the factory welds. Didn't even get to make 1 pass. TJ did a good burnout, then did(tried to do) 1 dry hop. The welds broke loose, sending the pinion yoke up thru the floor. :eek:

After that, we never made a pass without adding some tack welds to each tube. I never welded 'em up solid. I've read that welding solid can pull the tube out of line, unless you have the rear in some sort of jig, or welding fixture.

I used a Lincoln stick welder with 7018 rods. Made 4 tack welds, 90° apart. The 2nd tack was 180° from the 1st. Did several 12-bolts this way. Never had any problem with any 12-bolt. But, my homemade 455's only had probably 400hp/500 torque. Best ET's were in the 11.80's.

Had one '68 Bird that actually made nearly a whole season with a Buick 8.2. If I remember correctly, it had 3.64 gears. But now, that number sounds odd. So, I might be wrong about the ratio. Anyhow, that car ran mid 12's, & won races.

But, then one nite in Tyler, TX, the pinion broke, which oiled down the track, and made others unhappy. We never used anything but factory axles & never broke one. But, I ran stock 13" converters in all the 455 cars. That might explain it.

If I am remembering the interchange-ability of 1st and 2nd gen 'F" body stuff....Wasn't 1st gen "F" body rear suspension used in Nova's till 1974?

And then the Nova rear suspension got the 2nd gen rear suspension when they changed body styles in 1975??? (which was a terrible decision...)

You may be thinking of front suspension. The X-bodies switched over to the front steer front sub-frame in '75. My '74 Ventura still had the rear steer sub-frame.

I don't know that the rear suspension was changed. BUT, some low power models came with a 7.5 rear end, rather than the 8.5.