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

[Miranthis]

When I search, I get lots of article on how to measure the car to get the ideal drive shaft length to order one. What I need to do is determine if the shaft i have is the one I need. The car in question came with and 8.75 from the factory, but had the rear end replaced long ago when the SG malfunctioned. It got a replacement "from the yard" as is a 7.25 rear. No one knows if the driveshaft was also swapped or if this is the original shaft and I know that the original driveshaft was 49.58 in. and was 2.75 dia. Questions is WHAT was 49.58 in.? Is that total length from yoke to yoke (X that I added in the drawing below), center to center (A in the drawing), or something else? I am putting an 8.75 back in and want to measure to see if this driveshaft works or if it was swapped out/cutdown, etc. I don't see any obvious evidence of shortening.

 

[Mopar Momzee]

you measure a to a. Also make sure car is on ground as you where driving it/ level.

 

[Big.J.007]

This method has not failed me yet

 

[Oldmanmopar]

What model car are you working on 108 wheel base or 111? 727 with and 8 3/4 for a 108 is 45 1/2 add 3 inches for a 111.

 

[doug 371]

a to a

 

[Dana67Dart]

The car in question came with and 8.75 from the factory, but had the rear end replaced long ago when the SG malfunctioned. It got a replacement "from the yard" as is a 7.25 rear. No one knows if the driveshaft was also swapped or if this is the original shaft and I know that the original driveshaft was 49.58 in. and was 2.75 dia

The drive shaft for a 7 1/4 is longer by about 1 1/2"

Look in the parts book for your car and the length and dia are listed for the car, the rear and the trans.

Some years of cars with 7 1/4 rears had a drive shaft with an isolator in the rear. Meaning the rear part of the shaft was small enough to fit inside the front part with rubber between the two

 

[Miranthis]

you measure a to a. Also make sure car is on ground as you where driving it/ level.

Ok, i understand why a car should be level for the "find the best custom length" measurement, which is exactly what I am not doing. But, to measure the distance A to A can be done off the car and with no u-joints installed, which is the case I have now. I want to measure the driveshaft length and determine which one I have.

 

[Miranthis]

The drive shaft for a 7 1/4 is longer by about 1 1/2"

Look in the parts book for your car and the length and dia are listed for the car, the rear and the trans.

Some years of cars with 7 1/4 rears had a drive shaft with an isolator in the rear. Meaning the rear part of the shaft was small enough to fit inside the front part with rubber between the two

Again, I know what length I need from the specs, what I don't know is measure from where to where. Is it center to center on the u joints, or total length of the shaft, i.e., length A or length X on the drawing. I am also seeing some folks measure from weld to weld (length Z).... for the same shaft those will be three different numbers.

 

[Mopar Momzee]

Yes no ujoints installed is fine but @Oldmanmopar gave you the measurements that i think you require. Otherwise you need to have everything bolted in and at drive height. The reason at ride height is so that when driving there’s play in driveshaft for flexibility and not hitting trans and busting everything up.

 

[Miranthis]

Yes no ujoints installed is fine but @Oldmanmopar gave you the measurements that i think you require. Otherwise you need to have everything bolted in and at drive height. The reason at ride height is so that when driving there’s play in driveshaft for flexibility and not hitting trans and busting everything up.

this is a solid drive shaft. Any play would be in the slip yoke, which again is something to account for if I was looking for the ideal or custom size needed. However, I not looking for the "how to measure for a custom drive shaft length" but rather for "how long is THIS SHAFT i have here on my bench". The spec is known for the factory shaft (49.58 inch) and I need to know if this one is or is not 49.58 inch. is the spec of 49.58 inch a measurement of A, B, X, or Z on the drawing, cause they are all different numbers.

 

[Miranthis]

What model car are you working on 108 wheel base or 111? 727 with and 8 3/4 for a 108 is 45 1/2 add 3 inches for a 111.

1966 Barracuda, 273, A-833, 8.75 Rear. Book shows part number 2781 952 book shows 49.58 inch, but the issue is that 49.58 is measured from where to where. Center of u-Joint to center of u-joint, weld to weld, or end-of-yoke to end-of-yoke?

 

[Vaanth]

1966 Barracuda, 273, A-833, 8.75 Rear. Book shows part number 2781 952 book shows 49.58 inch, but the issue is that 49.58 is measured from where to where. Center of u-Joint to center of u-joint, weld to weld, or end-of-yoke to end-of-yoke?

The measurement is made from the center of the first universal joint to the center of the other universal joint, which is dimension A in your drawing.

 

[H22A4]

Does anyone have any advice on what the A-B measurement should be?

 

[318willrun]

1966 Barracuda, 273, A-833, 8.75 Rear. Book shows part number 2781 952 book shows 49.58 inch, but the issue is that 49.58 is measured from where to where. Center of u-Joint to center of u-joint, weld to weld, or end-of-yoke to end-of-yoke?

Everyone that I know (including me) talks eye to eye when talking lengths of drive shaft.
So yes, center of u-joint (eye) to center of u-joint (eye) gives you the length of drive shaft you have. Just like what "A" shows in your first diagram

 

[Dana67Dart]

With your drive shaft out of the car, measure A to A (center of one u joint to the center of the other u joint)

That is the ONLY way drive shafts are measured.

I think many are telling you how to measure FOR a drive shaft which is not what you are asking.


This is the stock 7 1/4 driveshaft for some years, (2 pieces with rubber between) if yours is currently like this most likely it is 7 1/4 length.

Not the shaft on the left bottom is shorter then the top one.

This is from 67 parts book, you will need to look for your car and years parts book.

Yellow is 7 1/4
Green is 8 3/4
Red is note about measuring center to center

 

[Ironmike]

My question is how much "play" should be in your yoke? I have very little, just enough to slip the U joint into the rear end.

 

[Dana67Dart]

When the rear is hanging it is closer to the transmission than when the car is on all fours

 

[Mojoe9955]

you measure a to a. Also make sure car is on ground as you where driving it/ level.

A to A will always be the same where ever it is...

 

[Ironmike]

Seriously, what is the desired play. I squish my rubber tranny seal just to get the u joint into the yoke. Maybe 3/4 inch play. Doesn't seem enough.

 

[H22A4]

Seriously, what is the desired play. I squish my rubber tranny seal just to get the u joint into the yoke. Maybe 3/4 inch play. Doesn't seem enough.

Yup, which is why I asked earlier what an approx A-B measurement should be.

 

[72bluNblu]

Seriously, what is the desired play. I squish my rubber tranny seal just to get the u joint into the yoke. Maybe 3/4 inch play. Doesn't seem enough.

This is a totally different question than what the OP is asking but you’re supposed to have ~1”. But that has nothing to do with the seal. You bottom the yoke out in the transmission, then slide it back 1” and measure for the driveshaft with the yoke in that position.

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.

 

[Dana67Dart]

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

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.