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Old 01-06-2013, 05:22 AM
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48 pontiac rear end

I have a 48 Pontiac that I plan on street rodding by putting in a sbc 350 and 700r4, making it more of a highway cruiser. My question is has anybody done this and used the stock rear end ? How strong are these rear ends ?
What size ring gear are these ? Any info ?
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Old 01-06-2013, 02:24 PM
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Quote:
Originally Posted by JeffE View Post
I have a 48 Pontiac that I plan on street rodding by putting in a sbc 350 and 700r4, making it more of a highway cruiser. My question is has anybody done this and used the stock rear end ? How strong are these rear ends ?
What size ring gear are these ? Any info ?
Thanks
Look at it this way....the most powerful motor that was used by Pontiac in 1948 was rated at 220 lbs/ft of torque. If your motor will make that torque or less, then use the original diff. If you plan more power than that, then replace the diff with a later, stronger unit. I believe the original Pontiac diff was hung on leaf springs and had an open driveline. The rear track (centerline of the passenger side tire to the centerline of the driver's side tire) was 61.5", so take a tape measure and find a boneyard donor that has measurements close to that with leaf spring pads on the tubes. Brakes were internal-expanding shoe, hydraulic type, so any later model diff that has the spring pads and ~61.5" track should work. I might start off looking at Gen I Camaros.

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Old 01-06-2013, 08:18 PM
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Jeffe, I'd be concerned about using the '54 and earlier GM enclosed driveline rearends in any V8 application. You would have to adapt the newer engine to an older trans that fit the enclosed driveline, or adapt a newer trans to the old enclosed driveline sytem.
Best to find a newer rearend that allows an open driveline, and not use your present setup. I would measure your spring perch centers, and the width of the rearend from drum surface to surface. You may most likely need new spring perches, so overall width is really the most important measurement. Then start looking around the wrecking yards to see what might interchange, and various bolt patterns. I'd guess your's is still 6 bolt, and you may have to either convert the front to match a 5 bolt patterrn, or use different bolt pattern for the rear.
An S10 rearend with 5 on 4.75" pattern often works well in these older Pontiacs width wise, and the 7.5" 10 bolt has been used behind V8's with good success.
Here's a rearend width chart for donor rearends:
Rear End Widths
Since you're going with a 700R4, you'll have overdrive, so try to find a S10 rearend (or whatever rearend) with a ratio around 3.50-3.73, as that will work best with the 700R4's low 1st gear, and overdrive.
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Old 01-07-2013, 06:28 PM
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Thanks guys for your advise. Jeff
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Old 01-07-2013, 06:42 PM
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I would also like to add that if you are adding more power the stock brakes seriously need upgrading anyway, front and rear.
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Old 01-08-2013, 01:44 PM
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Be careful when using charts that list "rear end widths" and pay close attention to where the measurements were taken from. They might have measured from drum to drum or from axle stub to axle stub or from the centerline of the rear tires (track).

I suggested the Gen I Camaro after finding the TRACK of the '48 Pontiac and the TRACK of the Camaro and the fact that the Camaro is leaf-spring suspended, just as in your car. I'm not saying that the Camaro is the only choice out there. As was suggested, get all the measurements off your car, then take a tape measure to the various boneyards in your area and begin your search.

Also, if you need to tweak the track a little to use a taller or wider tire on the back of the car, wheels are manufactured with any range of backsets.
Wheel tech - Wheel offset clarified
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