Anyone know how this compares on a GM rear end? - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 12 (permalink) Old Jul 7th, 20, 1:24 PM Thread Starter
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Anyone know how this compares on a GM rear end?

Anyone know if there is a difference from the axle centerline to the center of the u-joint towards the front of the car between a 10/12 bolt chevy and a 9" ford? I know the ford is lower - Looking for the fore/aft dimension.

I'm just wondering if I need a new driveshaft.
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Tighten it till it strips and back it off a quarter turn.
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post #2 of 12 (permalink) Old Jul 7th, 20, 1:29 PM
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Re: Anyone know how this compares on a GM rear end?

Anytime the ring gear diameter is increased the centerline of the pinion is lowered.

I believe depending on the yoke used 10 an 12 bolt rear ends can use the same drive shaft.

I have no input on a Ford 9 inch.

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post #3 of 12 (permalink) Old Jul 7th, 20, 1:51 PM
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Re: Anyone know how this compares on a GM rear end?

Speaking of drive shafts, what do you have now?

If you're still using the factory shaft with 1310 U joints and you switch to a 12 bolt, where is the weakest link now?

A drive shaft failure can be way more catastrophic than a differential failure

Just food for thought. I know that snowball just keeps getting bigger.

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post #4 of 12 (permalink) Old Jul 7th, 20, 2:08 PM Thread Starter
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Re: Anyone know how this compares on a GM rear end?

Just FYI - I am looking to go from a 10 bolt to a 9" and wondering if the existing driveshaft will work.
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Tighten it till it strips and back it off a quarter turn.
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post #5 of 12 (permalink) Old Jul 7th, 20, 2:31 PM
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Re: Anyone know how this compares on a GM rear end?

You may have to measure yourself, or get the dimensions direct from a manufacturer of 9"s off their blueprints. Axle centerline to centerline of ujoint in yoke. Which you may have to do yourself for the GM unless someone has, and jotted it down.

PS I have a Denny's HD and "foolishly" leave greaseless 1310s in for the weak link. Safety loop for sure. And it does chew them up, esp the rear. ( which I dont have a loop for hut could easily weld one in near the mongo mufflers) !)

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post #6 of 12 (permalink) Old Jul 7th, 20, 2:32 PM
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Re: Anyone know how this compares on a GM rear end?

I just googled it. It gets a little confusing, but could possibly work.

Are you having a rear end built or buying a used unit?

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post #7 of 12 (permalink) Old Jul 7th, 20, 2:53 PM
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Re: Anyone know how this compares on a GM rear end?

There are different length yokes for the 9 inch, so depending on which yoke you use, your drive shaft may be close or it may be a few inches off. I used the long car yoke for a while in mine with the stock drive shaft. It's best to measure and do the math. That way you don't get into trouble and possibly destroy your car.
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post #8 of 12 (permalink) Old Jul 8th, 20, 8:51 AM
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Re: Anyone know how this compares on a GM rear end?

So no one knows the answer to the original question?
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post #9 of 12 (permalink) Old Jul 8th, 20, 10:01 AM
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Re: Anyone know how this compares on a GM rear end?

Even if you knew the dimension it wouldn't help a lot because of the lower pinion center line. When you lower the pinion it will increase the length required for the drive shaft. The only way to know what length of drive shaft you need is to put it together and raise the rear end to ride height and measure. http://www.strangeengineering.net/wp...Order-Form.pdf
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post #10 of 12 (permalink) Old Jul 8th, 20, 11:38 AM
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Re: Anyone know how this compares on a GM rear end?

Quote:
Originally Posted by big gear head View Post
Even if you knew the dimension it wouldn't help a lot because of the lower pinion center line. When you lower the pinion it will increase the length required for the drive shaft. The only way to know what length of drive shaft you need is to put it together and raise the rear end to ride height and measure. http://www.strangeengineering.net/wp...Order-Form.pdf


Yes Sir Freddie. On the 70 a few months ago I went from a 10 bolt to 9". 9" pinion is almost 1"inch lower and 1.25" shorter. As you stated, exact reason why I waited to measure for a new shaft after the rear install. Too many variables to just go off measurements. For example if going off just measurements would have required about 1.5-2" longer drive shaft. My 70 is lowered 3" (another reason why I like the 9" lower pinion) in the rear which moved the rear end higher in the fender well ultimately pushing my original shaft (if bolted to the rear pinion) forward towards the Trans. Also, since going with a bigger engine and the 9", I went with 1350 joints. All that combined and pinion angle set, I ordered a shaft only 5/8 longer. Stance will also play a part.


As stated by Freddie, get the rear and install. Adjust for Trans to rear pinion angles and with the car fully weighted on the floor, THEN MEASURE.


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post #11 of 12 (permalink) Old Jul 8th, 20, 2:52 PM
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Re: Anyone know how this compares on a GM rear end?

Yes, the pinion angle must be set correctly before measuring for the drive shaft. Changing the pinion angle can make a bid difference in the drive shaft length.


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post #12 of 12 (permalink) Old Jul 8th, 20, 2:57 PM
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Re: Anyone know how this compares on a GM rear end?

depending your current trans yoke , a longer yoke might be available so original driveshaft could be retained
But due to the number of variations of Ford pinion yokes, you cannot determine this length without all installation complete and measured exactly as mentioned previously

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