# open to posi swap questions



## ylwgto (Oct 27, 2009)

Finished my engine refresh, now on to getting rid of the one legger!

I have an open diff in my 8.2 10 bolt in my '66. Not sure what the gear ratio is. I'm going to do a 4 wheel disc conversion, so thinking i may as well upgrade to a posi carrier while the axles are out. a few questions:

1. is this a driveway mechanic level job or are there nuances that make having this done at a shop better (i know there are details with the carrier bearings)?
2. i have a '68 400 with a mild cam and a pretty tired TH400. What gear ratio would you all recommend (I'll be converting the car to a manual 5 speed in the VERY distant future)? Never race it and I'm trying to get it back to daily driver reliability. I do like burnouts, but I also like highway cruising below 3500 
3. brands of carrier recommendations? 

Thanks all.


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## armyadarkness (Dec 7, 2020)

I just did the same job on my 67. I had never done a BOP rear before, so it was my first. I bought an Auburn Posi to replace my open diff, and I reused the existing 3:36 gears. I ordered the posi, install kit, and axle bearings and seals. Did it on the ground in the driveway in about 3 hours. Not bad for having never done it before.

Key notes: Have a gear lube pump before hand (because squeezing in 90wt sucks and never works well), verify that the axle bearings are correct (I got mine from a reputable seller and manufacturer and the were incorrect), have Permatex Ultra Grey (and it won't leak, even without a gasket), think it through (now's the time to convert the brakes and add a high capacity aluminum diff cover with fill ports!).

I use an angle grinder to cut off the lock rings and bearings, because I always replace them.

Two days after my job was completed, I felt that I could shim the diff even better, so I took it all apart and added another shim. This time, it only took an hour. It was intimidating when starting, but a fun job in the long run.


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## armyadarkness (Dec 7, 2020)

And I also have a 400/400. I do recommend 3.36. It'll light both tires through first and well into second. Napa had the bearings, lock rings, and seals.

Beware if you do a Google search for BOP/ Pontiac GTO axle bearings. These come up a lot on ebay and Amazon, from Moser, Youkon, Car id, and they're completely wrong.









Here's the diff I got.









GM 8.2" BOP Auburn HP Posi Differential 3.36-Up 5420113







www.ronsmachiningservice.net


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## ylwgto (Oct 27, 2009)

Thanks ARMY. Great info here. I watched a vid online and I don't think my skills are up to the job. I'm doing a disc swap, so maybe I'll change my mind when those axles are out!

I think 3.36 sounds good and I was also eyeballing Auburn units. Appreciate the info.

Are our stock 10 bolts 28 spline axles?


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## deanhickey (Apr 6, 2015)

ylwgto said:


> Thanks ARMY. Great info here. I watched a vid online and I don't think my skills are up to the job. I'm doing a disc swap, so maybe I'll change my mind when those axles are out!
> 
> I think 3.36 sounds good and I was also eyeballing Auburn units. Appreciate the info.
> 
> Are our stock 10 bolts 28 spline axles?


If you stick with the current gears you gan get away without messing with the pinion gear(I think) and that will save you a lot of time. A dial indicator with a magnetic base is a handy tool for this job, it will allow you to check the backlash on the ring gear when you reinstall it. That way you can nail it on the first go and not have to drain and refill the rear end, which is the worst part of the job as far as I am concerned. Good luck with the project and let us know how it works out for you.


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## ylwgto (Oct 27, 2009)

deanhickey said:


> If you stick with the current gears you gan get away without messing with the pinion gear(I think) and that will save you a lot of time. A dial indicator with a magnetic base is a handy tool for this job, it will allow you to check the backlash on the ring gear when you reinstall it. That way you can nail it on the first go and not have to drain and refill the rear end, which is the worst part of the job as far as I am concerned. Good luck with the project and let us know how it works out for you.


good point. not sure what gearing i have in my current rear but the stock open diff i grenaded a decade ago was 2.73...way too for a 3 speed auto by my preferences!


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## deanhickey (Apr 6, 2015)

ylwgto said:


> good point. not sure what gearing i have in my current rear but the stock open diff i grenaded a decade ago was 2.73...way too for a 3 speed auto by my preferences!


Have you tried jacking the rear wheels up and counting the number of rotations of the driveshaft per rotation of the tire? This should give you a ball park idea of what you have.


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## ylwgto (Oct 27, 2009)

deanhickey said:


> Have you tried jacking the rear wheels up and counting the number of rotations of the driveshaft per rotation of the tire? This should give you a ball park idea of what you have.



will be doing that in the near future. just got the car off jack stands so I'm gonna hold off for a few days and drive it!


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> Thanks ARMY. Great info here. I watched a vid online and I don't think my skills are up to the job. I'm doing a disc swap, so maybe I'll change my mind when those axles are out!
> 
> I think 3.36 sounds good and I was also eyeballing Auburn units. Appreciate the info.
> 
> Are our stock 10 bolts 28 spline axles?


If you ask me, the videos are often, very "over complicated". As far as I'm concerned, you can set a diff up by feel... for the most part! Especially if you're keeping the original gears. Yes the tools of the trade can speed and simplify things, but ironically, they're often only required for unskilled mechanics. In every case that I've ever seen, you can simply swap out the old for the new.

However!!!! That is not a rule, and if it does go sideways, that's when skill and tools come into play. Diffs are super intimidating, but not really as complicated as they appear on youtube. Too tight and they'll get hot and wear, too loose and they'll clank or break. But unless you're making well over 400 HP, and using it regularly, then you CAN do your diff with confidence!

The most common thing I see go wrong on a diff rebuild; is finding out that you need a part you don't have, and then you need to stop work and go get it, or worse, order it. But if the car is a project and you have time, it's no big deal. 

Typically, if you reuse the original shims, in the same locations, then the diff will work perfectly, without any further setup. If you're just doing the posi and going to reuse your existing gears, then you won't even need to pull the pinion at all. And unless it's sloppy or noisy, there's no need to. 

Don't let me talk you into it, but whatever you do... don't let Youtube talk you out of it.


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## armyadarkness (Dec 7, 2020)

deanhickey said:


> If you stick with the current gears you gan get away without messing with the pinion gear(I think) and that will save you a lot of time. A dial indicator with a magnetic base is a handy tool for this job, it will allow you to check the backlash on the ring gear when you reinstall it. That way you can nail it on the first go and not have to drain and refill the rear end, which is the worst part of the job as far as I am concerned. Good luck with the project and let us know how it works out for you.


Agreed with draining... You lose all of your time removing, draining, cleaning, and refilling!


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## armyadarkness (Dec 7, 2020)

ylwgto said:


> will be doing that in the near future. just got the car off jack stands so I'm gonna hold off for a few days and drive it!


Gearing is another issue... Stay away from hype! Use what you love. 2.73 may be a joke to some, but if it's what you need/want, then do it! 

I just did front and back gears on my Jeep Wrangler with 35" tires. I told everyone that I was putting in 5.13's, and they ALL (even seasoned mechanics) thought that I was crazy. I got almost 6 more miles to the gallon and the car id a joyful, daily driver. Tall, heavy tires require low gearing. It's what worked for me, you, whomever. 

Pontiac engines make big torque, so you don't need the 3.73's that a Chevy guy puts in his Camaro with a 305


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## armyadarkness (Dec 7, 2020)

deanhickey said:


> If you stick with the current gears you gan get away without messing with the pinion gear(I think) and that will save you a lot of time. A dial indicator with a magnetic base is a handy tool for this job, it will allow you to check the backlash on the ring gear when you reinstall it. That way you can nail it on the first go and not have to drain and refill the rear end, which is the worst part of the job as far as I am concerned. Good luck with the project and let us know how it works out for you.


Yep. No need to touch the pinion. However, in every case that I've seen, if you reuse the original pinion shims, it has worked fine. I can tell you, I didn't touch my pinion, but I still have some noise in the car that I don't like, and since the PO completely neglected the car, I'll likely pull it all apart again and replace the pinion bearings and seals. I already have them, so it's just not worth skipping them... Even though I really want to just be DONE!!!! NO SHORT CUTS! 

Some day I'll be happy that I did it right.


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## deanhickey (Apr 6, 2015)

armyadarkness said:


> Gearing is another issue... Stay away from hype! Use what you love. 2.73 may be a joke to some, but if it's what you need/want, then do it!
> 
> I just did front and back gears on my Jeep Wrangler with 35" tires. I told everyone that I was putting in 5.13's, and they ALL (even seasoned mechanics) thought that I was crazy. I got almost 6 more miles to the gallon and the car id a joyful, daily driver. Tall, heavy tires require low gearing. It's what worked for me, you, whomever.
> 
> Pontiac engines make big torque, so you don't need the 3.73's that a Chevy guy puts in his Camaro with a 305


2.73:1 is a great highway gear, I have a 3.36 in the car at the moment but have a separate complete reared with 2.73 gears. Will use that rear for a ross country trip in a few years.


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## ylwgto (Oct 27, 2009)

armyadarkness said:


> Agreed with draining... You lose all of your time removing, draining, cleaning, and refilling!


So I was looking at aluminum rear covers with drain holes and they usually have the girdles built in. overkill for my application, but I'm assuming they are ok to use with stock rears?









BUICK OLDS PONTIAC 8.25 10 BOLT TYPE BOP AXLE ALUMINUM COVER PERFORMANCE GIRDLE | eBay


It fits 1964-71 10 Bolt 8.25" (type B/O/P), commonly found in Buick, Olds & Pontiacs. Type B/O/P 10 bolt rears have press-on axle bearings and the cover gasket shape appears "notched". Maximize gear life and performance with this LPW ULTIMATE support cover.



www.ebay.com


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## armyadarkness (Dec 7, 2020)

I love aluminum diff covers. They work like a heat sink, so they drop temps, they usually add capacity, which also drops temps and prolongs life, and they have substantially more efficient draining and filling. so long as it's BOP, then go for it. Even with stock, there's big room to improve.


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## ylwgto (Oct 27, 2009)

Wondering if anyone can help with IDing my rear end

On center section: 
J85
9783393

On passenger axle tube:
WB

On Wallace Racing the "WB" axle code is for a 67 Tempest with 2.56 open rear. But my center section casting (J85) suggests Oct 8, 1965 (I think)???
Am I doing this wrong?


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## ylwgto (Oct 27, 2009)

armyadarkness said:


> I love aluminum diff covers. They work like a heat sink, so they drop temps, they usually add capacity, which also drops temps and prolongs life, and they have substantially more efficient draining and filling. so long as it's BOP, then go for it. Even with stock, there's big room to improve.



Well, I bit the bullet and bought the Moser rear out of a pals '66 Chevelle. Just installed the rear discs and am trying to get the rear into the car next week. 
So, soon there will be a '67 BOP 10 bolt for sale!


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