What are different from 10 and 12 bolt posi rear ends?

2 bolts.

Kidding aside - one needs to be a bit more specific.

There are our 10-bolt 8.2 inch Pontiac axles, 10-bolt 8.2 inch Buick axles, Chevy 10-bolt 8.5 inch axles and Chevy HD 12-bolt axles.

There are total width end to end issues (as the 8.5 and 12 bolts come in several varieties) and compatibility with your factory rear springs (spring perch type and location).

You might want to try some searching in the archives here.

If you really want to be confused, here is a lot of info I pulled from the Performance Years Forums a couple of months ago. I most concerns the Pontiac HD 8.2 4-pinion rear axle, but talks about other GM ones:

All factory 64 & 65 Pontiac 8.2 Safe-T-Track & Olds 8.2 Anti-spin units were 2 pinion units. All '64-65-66 model production 8.2 Pontiac housings were gray iron... very susceptible to pinion deflection under high torque loads. Pinion deflection is big reason why all gray iron GM 8.2 rearends blow up when exposed to heavy torque loads, no matter what posi carrier (rebuilt 4 pinion, eaton, auburn) & good aftermarket axles are installed in it. The weakest gray iron housings ever cast were used in '64-66 Olds & Buick 8.2 rears, they are junk.

4 pinion cone type 8.2 Pontiac Safe-T-Track carriers were introduced for '66 models with the low ratio 3.36-4.33 gearsets. They were also used by Oldsmobile in '66. Pontiac dealers were advised to replace destroyed '64 & 65 two pinion Safe-T-Track units with the 8.2 HD 4 pinion Safe-T-Track carriers.

If one has a Pontiac 8.2 2.56 or 2.78 Safe-T-Track rearend, it is a 2 pinion, as no 4 pinion units were made for the high ratio (2.56 & 2.78 rears).

Mid ratio 4 pinion Safe-T-Track units were produced in early (through '69) & '70-72 8.2 Pontiac styles. The mid ratio 8.2 Pontiac carrier was used with 2.93, 3.08, & 3.23 8.2 Pontiac gearsets. While there were many 4 pinion Safe-T-TRACK rears built with gray iron housings, there never was a 8.2 HD SAFE-T-TRACK rear built with the mid ratio gears & 4 pinion carrier.

To qualify as HD SAFE-T-TRACK, the 8.2 Pontiac rear was built with the nodular housing and the 4 pinion carrier AND the special forged axles. Tons of '67-72 Pontiac 3.55 SAFE-T-TRACK rears were built with nodular housings, but with the the standard duty '67-69 or '70-72 Pontiac 8.2 axles.

Buick built their own HD 8.2 BUICK 10 bolt rear from '68-70. It takes it's own gearsets (like their 2.93's, 3.08's, 3.42's & 3.64's) & its own different series 4 pinion posi carriers. These Buick 4 pinion 8.2 posi carriers had different spline side gears & their own stagger for different ratios. If one is not careful buying used parts they can end up with a 4 pinion Buick posi carrier for intended Pontiac 8.2 usage.. The '68-70 Buick 8.2 posi unit & single track carrier takes a 12 bolt carrier bearing on the drivers side & normal 10 bolt carrier bearing on the passenger side

It's really not a mess, its actually easy to understand, if you look at it as a continuation of improvements as power levels rose as the torque loads increased with more powerful engines. Through the 60's GM had 4 divisions building A-bodys in the US, & each division basically had its own gear & axle assembly at least enough autonomy that in '68, 69, & 70 each division had its own rears. GM, Ford, Chysler all increased the strength of their rearends in their highest performance vehicles as the decade rolled on. Warranty claims is not what they wanted, so they upgraded... at least behind their performance engines.

In GM's case, the common Chevy 12 bolt was not the General's strongest factory A-body or F-body rear. Not unless you happened to to have been one of the very few who ordered a COPO Chevelle, an L78 or LS6 Chevelle, or COPO Camaro, or an over the GM counter version of one the truly HD 12 bolts for those cars. The rest of Chevrolet Divisions Chevy 12 bolt offerings, the vast majority of Chevy 12 bolts built had weak axles coupled with their really weak link: the brittle spider gear/side gear combination. Truth be known, the 30 spline standard duty Chevelle 12 bolt axles lack a lot in strength to the best bolt-in 10 bolt axles, thus counting axle splines really won't tell the whole story. In the case of factory Pontiac 8.2 axles, it wont tell if a 28 spline sealed bearing axle is a regular light standard axle or if is is one out of HD SAFE-T-TRACK rear. As far as metallurgy of the 12 bolt Chevy center housings I have not studied that a ton. Would love to compare a 69 BE coded Camaro 12 bolt to the '69 BV I have in my racks. Both are 4.10 positraction Camaro units, the BE has all the good stuff & has a different casting number on the hsg. Nodular iron, a good bet.

More on axles... the bolt in axles out of '64 or 65 Olds Cutlass or 442 or Buick Skylark or '65 GS will bolt right in a '64 or '65 Pontiac 8.2 rear, and are the same strength std duty axle. Now what about the fellow who just bought a early GTO or LeMans project car & over the previous years, the Pontiac's original rear was swapped with the weak early Buick 8.2 out a '64-66 Cutlass or '64-66 Buick Skylark. They look nearly identical, but the strength of the gray iron center section early Buick 8.2 is not near as strong as the "weak" gray iron Pontiac 8.2 housing out a '65 Tempest, LeMans or GTO. Olds even specified the Pontiac 8.2 in many of its higher torque application '66 442's. that housing was just Pontiac's standard '66 gray iron 8.2 housing rear. Not the nodular 8.2 Pontiac housings which first started showing up in '67 Pontiac A-body 3.55 Safe-T-Track applications. The nodular hsg 8.2 was really needed behind really strong applications like the '66 L79 W30 442. Olds instead dropped the 8.2 & developed the type "O" 10 bolt rear which lasted a few years.

Here was a good rear end discussion from way back:

10-bolt discussion

Wow, you really got Salmon going. To answer the question I think you're asking, the BOP 10 bolts in our birds is a pretty stout rear end. It's generally the same rear they used in the high horse A bodies before 1970. It's tougher than the chebby 10 bolt a little less tough than a chebby 12 bolt.

Our Pontiac 10 bolts are bolt in axles Chebby 10 and 12 bolts are C-clip

I want a posi rearend what should I go for for my 69 firebird....not sure about gears since im.not sure wat tranny I want toget

Why not just install an aftermarket posi unit in your present rear end?

8.2 BOP Posi Units

I second Salmon. There is nothing wrong with the BOP 10 bolt rears that came in our birds.

Stronger than the 10 bolts on the sister c-clip Camaros'

So put after market posi on my rearend any cons by doing this?

Nope