7 1/4 to 8 3/4 conversion

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rcs

rcs

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First off a Merry Christmas and a happy New Year to you all!

I am in the process of putting a 8 3/4 in my 64 Valiant to replace the 7 1/4 that has finally given up the ghost. The question is how long to make the drive shaft. I have seen a difference of 2.26" between the length of the two, but I'm wondering if this is good enough measurement to have the the drive shaft shortened by. I will not be able to get under the car to take a tape to it till spring, and would like to get the shaft ready to go before then. Thanks in advance for any help you can give.
 
Do you have the trunion or the slip yolk if it's a slip yoke you need to look and see how it's wearing coming out of the transmission it maybe too far out maybe too far in
 
Your combination was never stock so unless someone has done it and you trust their measurements.

You would be better to wait and measure twice. It only took my drive line company 3 days to turn my drive shaft.
 
Length will depend on your transmission- a factory '64 used a ball and trunnion outut shaft, later transmissions used a slip yoke; a 727 is longer than a 904, and both are different from a manual transmission- so it depends on what parts you've already got in the car, original or swapped.
Looking in my factory manuals, the specs for cars that have 7 1/4 and 8 3/4 rears differ by about 2" with all other equipment being the same; so if you have a driveshaft that was in it and working with the 7 1/4, shortening it by about 2" would be ABOUT right... but as others have said, the right way to do it is to install the rear and then measure for the new shaft, using whatever method your driveshaft shop wants you to use.
Also be aware that with this swap, you'll need new shock plates and U bolts, since the axle tubes on the 2 rears are different diameters. You also may have to deal with different U joint sizes, depending on how the new rear is equipped.
 
If you’re using a 904 slip joint trans with your 8 3/4”, I can measure my driveshaft when I get home. It’s out of the car and on the shelf.
 
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