Driveshaft length tolerances +/-?

Transmission and Drivetrain Tech

  1. kursplat

    kursplat FABO Gold Member FABO Gold Member

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    pretty sure most of us just use a tape measure. so a 1/4" off is probably not uncommon. i would want a 1/4" shorter. i'd rather loose a little spline engagement than have the yoke hammering the back of the trans
     
  2. kursplat

    kursplat FABO Gold Member FABO Gold Member

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    funny thing, when i added the 8 3/4, and had the new shaft built, they left it almost 1/4" longer than i asked for. went to install it and couldn't get the U-joint past the rear yoke. realized the rearend was just hanging on the springs, (which is not how i measured it). jacked it up a little and it fit fine. that worried me, so i loaded and unloaded the suspension as much as i could to make sure the yoke didn't bottom out. they got it spot on when i had it cut again for the gear vendors though. now i have a 1/2" of slip with the car at ride height
     
  3. SGBARRACUDA

    SGBARRACUDA ROY FABO Gold Member

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    Wait, are we still talking about automotive driveshafts?
     
  4. kursplat

    kursplat FABO Gold Member FABO Gold Member

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    uh, ya, why?...what are you talking about? :lol:
     
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    • Righty Tighty

      Righty Tighty FABO Gold Member FABO Gold Member

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      That’s kinda what I was getting at. Like, are we wanting accuracy down to a machinist’s standards, or a rough framer’s standards? Nothing wrong with either, yet a very big difference.
       
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      • pishta

        pishta I know I'm right....

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        I heard 1/4" stick out at minimum distance to pinion wherever that is in the spring/4 link travel. Farmer tolerances here.
         
      • Righty Tighty

        Righty Tighty FABO Gold Member FABO Gold Member

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        You’re gonna have to break that down a little so I can understand that.
         
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        • CudaFactHackJob

          CudaFactHackJob Well-Known Member

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          I like to get my yoke in the tranny as far as possible without bottoming out. I've had some so long that I had to remove the caps to get the rear u-joint into the rear yoke, then slip the caps on and tighten it down; still no probs with bottoming. The farther back that front yoke is from the trans, the more prone to vibration they are.
           
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          • RustyRatRod

            RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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            Make the measurement then subtract 1" for slip yoke stick out. That is the industry standard and has been for probably ever since cars have had slip yoke drive shafts. That's how I've always done it and they come out perfect every time.
             
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            • RustyRatRod

              RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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              Do that on a 4x4 that has a lot of rear suspension travel and you'll end up breakin stuff.
               
            • CudaFactHackJob

              CudaFactHackJob Well-Known Member

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              No kiddin...
               
            • RustyRatRod

              RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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              Same goes for a Mopar race or hot street rear suspension. They have a lot of travel, especially with XHD or SS sprAngs.

              Or shall I rephrase that? They're "SUPPOSED" to have a lot of travel. A lot of people just don't know how to set them up.
               
            • CudaFactHackJob

              CudaFactHackJob Well-Known Member

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              No they don't.
               
            • SGBARRACUDA

              SGBARRACUDA ROY FABO Gold Member

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              I’ve never seen this subject beat so damn hard. I think my mentor taught me back in 1969 to start with a yoke, leave “ of play at the trans and build the dimension that is left. Hell you think it’s tough now, you to to build a shaft with a trunnion joint.
               
            • Righty Tighty

              Righty Tighty FABO Gold Member FABO Gold Member

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              I apologize if this thread is beating a dead horse. I always try to do research and search here before asking questions, but I couldn't quite find what I was looking for. I was hoping to be able to find a driveshaft that would work without having a shop build one or cut one down because it seemed like a simpler process and could likely save money. So, I found the dimension for my car with the combo I'll have, and was wondering if there was a tolerance to that particular dimension, but it looks like the consensus is that I should measure anyway.
               
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              • Bad Sport

                Bad Sport HALF A BUBBLE OFF Staff Member FABO Gold Member

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                I swapped my rear out from a 7 1/4 to an 8 3/4 in my 74 Dart Sport. Using the original DS and taking the measurements that the shop I chose asked me to take I ended up with 48.75 inches CL to CL on the caps.

                The bill was a C note, they cut, balanced and checked it for run out, fit great and have had no issues.
                 
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                • Righty Tighty

                  Righty Tighty FABO Gold Member FABO Gold Member

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                  That’s much less than I was expecting. Obviously each shop will be different, but I was thinking more like $200-500 for something like that.
                   
                • jos51700

                  jos51700 Green Bearing thread connoisseur

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                  So my Dart has SS springs (two righties), and it's sky-high in the back. I want to de-arch them about 1.5-2".

                  You reckon I'll need to recut the shaft?
                   
                • Dartnut

                  Dartnut Don't hate me because i'm beautiful

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                  Boy, you guys tend to over think things.
                  A lot of people forget the fact that when these cars were built, there was sometimes a factory variance of up to 1/2'' in some cases for body squareness and that affects the rear end's position in the car. FROM THE FACTORY.
                  That's why the engineers built in a ''tolerance'' for drive shaft length. (As well as other components in the car)
                  So, unless you are Ronnie Sox or some other racer that has modified the car substantially from stock, then using the tried and true method of loading the rear suspension of the car to where it would be at rest, installing the yoke in the transmission and bottoming it out, measuring center to center of the U-Joints in the rear end and yoke, taking that number and subtracting 1'' will work just fine.
                  When the suspension compresses or you load 600 pounds of groceries in the trunk, the drive shaft yoke will slide into the tranny a bit, and when you jump your car like the Dukes of Hazzard and unload the suspension completely until it tops out, the drive shaft yoke will telescope out of the tranny a bit, but it won't fall out.
                  Pretty easy IMHO and 1/4'' either way won't be the end of the world in this case.
                  You're not building a Swiss watch here where the tolerances have to be within a billionth of an inch.........
                   
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                  • Dartnut

                    Dartnut Don't hate me because i'm beautiful

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                    No.
                    You are not changing the position of the rear end, or altering it's range of motion substantially.
                    That is of course taking for granted that it is the right length to begin with.........
                     
                  • mopowers

                    mopowers Well-Known Member

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                    You've got this backwards. When the suspension sags, the driveshaft actually moves towards the transmission slightly. Mine was a difference of 1/8" at full droop. This is because the the rearend pivots on the front leaf mounts, bringing it forward. When the rear compresses, the rearend moves backwards slightly. Still not significant enough to make a difference though.
                     
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                    • Dartnut

                      Dartnut Don't hate me because i'm beautiful

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                      Thank you, you are absolutely right!
                      I stand corrected.
                       
                    • kursplat

                      kursplat FABO Gold Member FABO Gold Member

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                      :lol: no allowance for a long holiday, and at least for me work, weekend with too much screen time and not enough real work to do?
                       
                    • CudaFactHackJob

                      CudaFactHackJob Well-Known Member

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                      And there's more to it than what we've covered so far... Some of these A bodies have the tire too close to the front of the fender so you might want to redrill the spring perch and move it back. Now's the time to do it if you're going to. I don't think the discussion is complete until all questions are answered. Some of us are picky on ds length...just because we haven't been as lucky as some..
                       
                    • DoctorDiff

                      DoctorDiff Well-Known Member

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                      As a rule of thumb, slip yoke travel (to bottom out) is 0.75"-1" when the vehicle is resting on its suspension.

                      The U-joint center to center measurement will also vary because slip yoke lengths differ.
                       
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