How to measure the drive shaft..... not "for the ideal driveshaft"

72bluNblu said

I don’t know what kind of seal you have, I’m not gonna assume they’re all the same. But if the yoke is compressing the seal when the driveshaft is installed the driveshaft is more than likely too long. My experience has been that if the rear seal is installed it has to be compressed significantly to bottom the yoke, but with the correct length driveshaft installed the yoke is not compressing the seal at all, there’s a gap before that happens. Again, I would not assume all the rear seals are the same

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I did an 8 3/4 swap.

The parts book says 52.58 (IIRC)

My trans has a flat seal, I replaced it with a bellows style.

Before the swap with the car on the ground I measured the distance the slip yoke was out of the trans.

After the swap with the car on the ground I positioned the yoke the same distance out of the trans and measured center to center and I got 52.25 (IIRC) I decided to split the difference and made the new shaft 52.375"

After the shaft was installed the bellows gust touches the fillet of the slip yoke when the car is on all 4s.

If I had the flat seal it would not have any interference.

Either way the yoke is out of the trans about 1.5 to 2"

Interestingly the OEM drive shaft from the 7 1/4 was also shorter than the measurement in the parts book.