why go 12 bolt? do a later 8.5" 10 bolt -alot cheaper and it was the H/D rear that became the standard and GM quit using the 12 bolt in the 71-72 years. find a 72-78 nova(75-78 8cyl cars have the 8.5 6cyl got a 7.5) posi's are all around though not cheap on ebay and there are alot of gear choices 3.23-3.42-3.73-4.10 that are OEM and reasonably priced. This type of rear held up fine in my 69 bird with a built 455/auto it ran 8.0 in the 1/8 mile - most 73 and later housings are multileafs and would be a good upgrade to your car too. Hope this helps you if you need any help I rebuild them e-mail is best as I'm not on this board often- rwhoag1969@verizon.net

 

for the pice they ar getting for the 12 bolt posi's you might consider upgrading to an
8.5 10 bolt they are extremely strong

 

The 8.5 10 bolt is stronger than the 8.2 10 bolt, but in stock form they are not a lot stronger. They need some upgrades to be stronger. The plug welds in the 8.5 axle tubes are very bad, so the axle tubes need to be welded. The 28 spline axles are the same as the 8.2 10 bolt, so that needs to be upgraded to 30 splines. A new 30 spline Eaton posi and 30 spline Moser axles should be first on the list of upgrades. When it is all done the only real difference between the price of the 8.5 and the 12 bolt is the cost of the housing.

 

the plug welds are problematic on all stock GM rears the main difference in strength of the 8.5 is the pinion is quite a bit larger than the 8.2 the stock 8.5 will get you into the 11's easily. welding the axle tube is a good idea if your gonna run slicks and juice if your not the stock 8.5 would be upgrade enough. the only people I have known to hurt one were running a healthy BB slicks and 200+ shot- upwards of 7-800 hp. my 3800lb 69 bird ran 8.0's all day long and never hurt the stock 8.5. but if your building one why not upgrade? depends on your budget.

 

Thanks for the response. I noticed another thread very similar to this one with more great info. It's definitely a big help - now I know what to look for.

 

The plug welds on the 8.5 are exceptionally bad. The 12 bolt has pretty good plug welds in it, but the 8.5 has a real problem with the welds. I don't know what happened to the 8.5, but the weld quality went into the toilet when they started making them.

 

I have to say I'm not in the racing end of it - I have seen 1 that the pumkin twisted but I dont know why they would be worse than any other - do they have less(plug welds) than say a 12 bolt? I've never compared the two, I thought they have 3 plug welds per side. possibly theres just lots more of them (10 bolts) at the strip than 12 bolts -and if the 12 bolt is at the strip maybe it had the tubes welded so you dont see it that often? just looking for a possible explanation to this.....

 

They both have 3 plug welds on each tube, but the weld quality is much worse on the 8.5 than the 12 bolt. It looks like the welder took his time and did a good job on the 12 bolt, but it looks like the welder was going as fast as he could on the 8.5. The welds are full of slag, cracks and many of them don't even fill the hole. I have seen some that looked like they didn't even connect to the tube. I have seen many 12 bolt and 8.5 rear ends and this is not just a few cases. The second gen Camaro owners have seen many of the tubes rotated in the housing.

 

For what it's worth...I swapped to an 8.5 inch posi rear (out of a '79 Skylark) about 7 years ago, and have never had a problem. I don't race it (except for one time in the 1/8mile at a dragstrip several years ago), but I do have a 450hp 383 stroker in front of it, with a TH400 and 3500 stall converter...

Hope this helps!

 

I don't ever plan on taking it to a track, so it shouldn't be a problem for me. I just didn't think the current rear could handle the power upgrade from a 327 to a 427.