2 pinion vs 4 pinion - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 11-08-2017, 09:52 PM Thread Starter
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2 pinion vs 4 pinion

I am new to the forum, and have been browsing around looking for Pontiac rear end info. I have a 68 400 Firebird and am trying to determine what I might need to rebuild, upgrade or simply replace the rear end. I have not disassembled it yet since I am planning on driving it a while before I jump into it. The only marking I can find on the axle tube is ZM. From what I find this is an indication of a 3.36, 8.2, 10 bolt posi. It also indicates it is a 4 pinion. I am not finding a whole lot of info on what that means. However, I did find one thread where they say it means there are six spider gears instead of four. This doesn't make sense to me at all. But I am not a rear end expert either. I see it was mentioned in a thread a few years ago about the 4 pinion, so I thought I would reach out and see if anyone could shed some light on the subject for me. Thanks!
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post #2 of 5 (permalink) Old 11-09-2017, 06:44 AM
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I'm no expert here, but I've never seen a 6 spider gear setup, nor can I imagine how one would work. My guess is that was a good example of forum 'help' offered from someone who didn't know, but I await someone showing me I'm wrong!
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post #3 of 5 (permalink) Old 11-09-2017, 10:55 AM
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Hopefully Pinion head will chime in here with his vast knowledge of these rear ends. All I really know is that the 4 pinion is somewhat stronger than the 2 pinion rear.
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post #4 of 5 (permalink) Old 11-09-2017, 01:30 PM
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The 4 pinion carrier is the HD limited slip version usually found on the higher output engine cars. Should also include the nodular iron housing which is supposed to be stronger than the standard grey iron units. Nodular housings would have cast number 9793020 for F body and 9793235 for A body. These housings use the Julian date codes vs the standard alpha character for the month+day of month method. The Julian cast date on the axle housing should be a lower number than the Julian date stamped on the axle tube (near the ZM code). The stamped date is the axle assembly date.

There would be 6 gears in the carrier. 2 side gears and 4 pinion gears. Original gears would also have the ratio stamped on the ring gear with the year. HD axle assemblies should have stronger axle shafts as well.

The shop manual shows the 3 different available carriers in some detail.

Hope this helps.

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post #5 of 5 (permalink) Old 11-09-2017, 03:11 PM
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The most common single track carrier used in any Salisbury style rearend utilizes a pair of side gears (where the axles slide in) along with a pair of pinion gears (aka Spider gears) that rride on a hardened tool steel shaft. The shaft is pinned from spinning in the carrier by a roll pin or pinion shaft bolt. Pontiac referred to their positraction units as Safe-T-Track (STT) carriers. The first 8.2 Pontiac STT carriers were of two pinion cone type design. These were avail in:

- high ratio (2.56, 2.78)
- mid ratio (2.93, 3.08, 3.23),
- low ratio (3.36-4.33...4.11-5.57 old aftermarket)

New for '66 model Pontiac 8.2' 10 bolt rearends with the optional Safe-T-Track carrier, was the low ratio 4 pinion STT carrier. Its design utilized a cross shape shaft for 4 Spider (pinion) gears, to ride in the cone type carrier. That particular low ratio 4 pinion Pontiac 8.2 STT carrier continued on in its use through the '69 model. In late '69, all Pontiac 8.2 ring gears began receiving 1/16" larger diameter LH thread ring gear bolts. This required slightly larger holes in the ring gear flange of the single track as well as STT carriers.

There was also a 4pinion mid ratio (2.93-3.23) STT carrier. The first one I've run across was pulled one from a very late built 3.23 STT '68 Bird rear. That stated, its use is somewhat odd. Typically this style carrier was found under 428 automatic '69 GrandPrixs, not A-bodys & Firebirds. Have had in my hands many original Firebird & Pontiac A-body 8.2 STT's of this time span, as well as have pulled over a dozen 4 pinion mid ratio STT carriers, out of '69 SJ's & one originally in my '69 428HO model J.

Within the '70-72 model years, the last incarnation of the Pontiac 8.2 rears were installed under 6 cyl, 350, & 400 Pontiac powered A-body's, as well as 400 powered GrandPrixs. The exception was '70 400 manual trans GP's received a McKinnion built 12 bolt, same as all '70-72 factory 455 equipped A-body & GP models.

As far as 6 pinion goes, I have not read the post, but there are many that throw around terms w/o knowing their*actual component name or use. I've heard or read differential side gears referred to many many times as "spiders", not spider gears, but are side gears... much larger in size, & machined to accept the spline end of an axle shaft. Another very common thrown around term is BOP 8.2 or BOP 8.2 10 bolt. Far from accurate. There were early Pontiac 8.2 rears & early Buick 8.2 rears. Each used its own housing & own axle. The early Buick 8.2 10 bolt are easy to spot had a very weak design cast center hsg. Oldsmobile also used the early Buick 8.2 under its '64 & '65 F85's/Cutlass/442 models & eventually came across the shortcomings in pinion support of the early Buick 8.2 design.. During the '65 model run, Olds began specifying Pontiac 8.2 cast & machined center housings to assemble housings for its lower ratio "Anti-Spin" 8.2 rears going in manual trans equipped 442's. This practice of Olds use continued through the '66 model 442's. When looking back, it's actually somewhat amusing, as the gray iron 8.2 Pontiac center housings had their own strength limits, but the early 8.2 Buick center housings were even weaker.

Nodular iron Pontiac 8.2 center housings were not introduced till the '67 model year. There are numerous different center housing casting numbers on these, with Firebird nodular 8.2 center hsgs getting their own casting numbers. I would expect the OP's 3.36 STT Firebird rear to have a nodular iron center housing, that however did not mean it was a HD STT rear/ that came with HD forged axles. Pontiac had a way of slowly stepping up the Ante when it came to the strength of their optional 8.2 Safe-T-Track rears. There is even a 4 pinion 8.2 single track carrier, that again began as a '69 GP deal. This factory Engineering/production process continued at least till the '70-71 time frame, when they decided with all the torque being produced by their top engines to go different directions.

Last edited by Pinion head; 11-09-2017 at 03:28 PM.
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