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Discussion Starter · #1 ·
I have a 68 convertible clone with a TH400 and a 2.56 open diff. I have found a 3.55 posi from a 67 GTO. Can anyone tell me steps to take to swap out the read end? a few years ago I installed boxed lower control arms and a rear sway bar so I've been at least that far into it. Any words of wisdom or warnings? Will I be able to use the same drive shaft?

Thanks
 

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The 65-67 rear ends are about 1/2 narrower overall on each side than the 68-72 rears. depending on your wheel backspacing and tire size, that may create rubbing issues for you. Other than that, assuming both rears are BOP 10 bolts, it should be a direct swap (same driveshaft yolk, control arm mounts, shock mount, spring perches and brake hook ups).

I just put a 3.25 rear in my 66 with a TH400 and its screaming at almost 3000 RPM on the freeway at 75...if you do HWY cruising just be ready for worse with 3.55! Great way to convince oneself to shell out for a 6 speed manual conversion 馃
 

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I prefer to pull the axles and hang the brake assembly up out of the way (Wire and/or Bungies),
avoiding any disconnection of the braking system in as far as hoses, etc.
Un-bolt Shocks, Upper & Lower Arms as well as the brake line on the upper center diff housing.
I swapped my '66 Differential with a '67 to test different gearing.
We had it swapped in under 2 hours.
 

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I have a '65 and it has flat spring perches, when did they start with the domed perches because I know they were there in '68, you would obviously notice that if you have seen both of the rear ends...just my 2 cents.
 

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Hi LouCabra,

As I understand the dimensions of the Pontiac rearend, 1964-'65 assemblies were 56 1/2-inch, narrower than the 1966 and up units. Your 67 rearend should fit fine in your 68.


Replacing a complete rearend housing is straight forward, requires standard tools and some mechanical aptitude. Take your time and take the necessary safety precautions. If you鈥檝e replaced the control arms you already know what鈥檚 required to loosen the control arm bolts. Best to use some penetrating oil to avoid bolt breakage.

Remove the drive shaft. Disconnect the rearend brake lines at the center rubber hose. Detach the emergency brake cables from the couplers that link the front cable to the rear. Loosen the rear wheel lug nuts, raising the rear of the car and resting the frame in front of the rear wheels on jack stands so the rearend hangs is helpful for getting at the top bolts. Remove the rear wheels. I remove the control arm nuts while leaving the bolts in until I rest the rearend on jack stands removing the weight from the bolts. You can use a floor jack to lift the rearend off the jack stands so you can remove the bolts and then lower it and remove the assembly from the car.

If not already replaced, now is a good time to replace the control arm bushings. The auto parts stores rent the tool to press them out/ in.

Reassembly is straight forward. Make sure that when you tighten the control arm bolts that the car/ rearend is as close to ride height as you can get it - then torque to factory specs.

Bleed the brakes.

Good luck.
 

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1967 400/400
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I'm not sure if I missed it, but why arent we just swapping out the carrier and gears, vs the the whole thing?
 

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I reread the post. I would take the carrier and gears out of the new housing and swap them into your existing one. Take the opportunity to swap your axle seals and bearings, which both diffs likely, already need anyway.
 

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The simplest swap would be removing the backing plates and brake tubing from the current rear and not open the hydraulic system as GTOJUNIOR mentioned, and just hang them out of the way. Then swap complete rearends and bolt backing plates back on and you're basically done.

While doing the carrier swap is tempting, there is a strong possibility that machining between the two center sections will be off enough to require resetting pinion depth and pinion/ring gear back spacing. For someone not familiar with setting up gearsets, swapping out complete rears is safer.
 

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Discussion Starter · #9 ·
Thanks for all the advise. First, I didn't know you could just swap out carriers. Unfortunately, I have two good reasons for wanting swap out the entire differential. First, I'm disappointed at the slow launching speed (feels like I'm driving a golf cart with the 2.56), and second, I'm pretty sure this car has been airborne at some point of its life. The right rear lower shock mount has been sheared off, not just broken, but sheared (can see the scrape marks), to the extent that when I installed new shocks I had you leave the existing shock port where it was and remove it from the new shock (I'll try to post a photo). I'll feel safer knowing for sure the shock will stay in place with a replaced diff.
As far as the gearing, I don't drive the car on the highway very much so I'g rather have some fun from the stop signs. I can always change the gearing to something more mild mannered once I get the new one in place.

Thanks again for the words of wisdom!
142446
Right-Rear_Shock-Mount.jpg
 

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Had a friend with a '67 that lost a wheel in traffic on the freeway at about 50 MPH. Wasn't a fun experience for him and he visited all four lanes but kept it off the walls. His damage to the shock mount was similar but about half of what I see in your photo. Yours looks like someone lost a wheel and then towed it home.

142449
 

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1967 400/400
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Thanks for all the advise. First, I didn't know you could just swap out carriers. Unfortunately, I have two good reasons for wanting swap out the entire differential. First, I'm disappointed at the slow launching speed (feels like I'm driving a golf cart with the 2.56), and second, I'm pretty sure this car has been airborne at some point of its life. The right rear lower shock mount has been sheared off, not just broken, but sheared (can see the scrape marks), to the extent that when I installed new shocks I had you leave the existing shock port where it was and remove it from the new shock (I'll try to post a photo). I'll feel safer knowing for sure the shock will stay in place with a replaced diff.
As far as the gearing, I don't drive the car on the highway very much so I'g rather have some fun from the stop signs. I can always change the gearing to something more mild mannered once I get the new one in place.

Thanks again for the words of wisdom! View attachment 142446 View attachment 142446
Far be it from me to talk anyone into anything. I don't know your skill level, but yes, the carriers are swapable. Also, the shock mount is easily fixable... However, it's your car, so do whatever you're comfortable with.
 

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I have a 68 convertible clone with a TH400 and a 2.56 open diff. I have found a 3.55 posi from a 67 GTO. Can anyone tell me steps to take to swap out the read end? a few years ago I installed boxed lower control arms and a rear sway bar so I've been at least that far into it. Any words of wisdom or warnings? Will I be able to use the same drive shaft?

Thanks
Have you looked on youtube?
 
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