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Old 09-19-2019, 02:58 PM   #1
72LB
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Driveshaft length question

I have a 69 shortbed 350/th400 combo and am on my third carrier bearing. I only have a 2.5/4 drop on my truck, does anyone have the measurements for a full length shaft?
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Old 09-19-2019, 04:57 PM   #2
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Re: Driveshaft length question

So many questions so little information. It is a factory short box? Coil or leaf springs? What rear end? Will the cross member hit the drive shaft? All these things affect your decision to convert to a one piece drive shaft. It would be better to have the measurement taken from your truck. If you're not comfortable making the measurements then a competent drive line shop can do it and then would be responsible if they came out incorrect.
Personally I would find the problem with what you have and fix it. It may take having your rear shaft modified and using an aftermarket carrier bearing. (Which is what I did on my Burban.)
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Old 09-19-2019, 05:07 PM   #3
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Re: Driveshaft length question

Factory shortbed, factory 12 bolt rear, coil spring. I've been told this combo came with either a full length driveshaft or a two piece thats why I'm asking.
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Old 09-19-2019, 05:50 PM   #4
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Re: Driveshaft length question

When I went from the TH400 to the the 700R4 the driveshaft shop had me measure the span from from the u-jojnt center line of both yokes bottomed out. He then deducted from that dimension to account for suspension movement.
I don't know if this helps but it is what I did and it works great.
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Old 09-19-2019, 08:39 PM   #5
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Re: Driveshaft length question

A 2.5/4 drop should be ok with a split shaft....

Biggest issue I have come across with center bearing failures is incorrect setup of the mount bolts...or the shaft is bottoming out in end of trans...

The center bearing carrier bolts should be loosely installed finger tight. The truck MUST be on all wheels and the rear suspension should be bounced up and down a few times....

Only then should the center bearing bolts be fully tightened...

There are also several aftermarket center bearing carriers available that seem to be more reliable than the stock units.

Finally, the rear shock angle changes substantially with a 4 drop...and raer shock relocation kits should be used to correct the shock angle...too much rear end bouncing around on a bad shock angle truck is sure death to a carrier bearing...

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Old 09-19-2019, 09:43 PM   #6
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Re: Driveshaft length question

I'm doing away with the carrier bearing! I've spent enough money on them. The shaft is not bottoming out on the trans and I know the trick for setting the carrier mount. As for the shocks there is no need to relocate them, if I had went with more than 4" drop then it is needed.

I'm surprised no one has a similar setup that has went to a full length shaft, everything I have found was with a th350 and not a th400.
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Old 09-19-2019, 10:04 PM   #7
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Re: Driveshaft length question

I dropped my coil sprung rear 4" and I absolutely needed the rear shock relocation mounts. It helps so the shocks aren't laying almost flat parallel and in a bind. You'll really like having them installed.

Gary
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Old 09-20-2019, 08:47 AM   #8
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Re: Driveshaft length question

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Originally Posted by GASoline71 View Post
I dropped my coil sprung rear 4" and I absolutely needed the rear shock relocation mounts. It helps so the shocks aren't laying almost flat parallel and in a bind. You'll really like having them installed.

Gary
That will go on the to do list, for now i'm really wanting to do the driveshaft swap.
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Old 09-20-2019, 12:23 PM   #9
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Re: Driveshaft length question

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Originally Posted by 72LB View Post
Factory shortbed, factory 12 bolt rear, coil spring. I've been told this combo came with either a full length driveshaft or a two piece thats why I'm asking.
I don't believe that any of our trucks came from the factory with a one piece driveshaft. I have heard of folks using an 80's vintage truck driveshaft. Do a search for driveshaft swaps.
I'm pretty sure I know why your eating carrier bearings but it sounds like your heart is set on the single shaft swap so I won't go into it.
Here is a link to a low buck shock mount modification to get more vertical shock mounts.

http://67-72chevytrucks.com/vboard/s...d.php?t=359399

Good luck!
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Old 09-20-2019, 07:15 PM   #10
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Re: Driveshaft length question

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Originally Posted by HO455 View Post
I don't believe that any of our trucks came from the factory with a one piece driveshaft. I have heard of folks using an 80's vintage truck driveshaft. Do a search for driveshaft swaps.
I'm pretty sure I know why your eating carrier bearings but it sounds like your heart is set on the single shaft swap so I won't go into it.

Good luck!
There were some shortbeds that had full length driveshafts but there behind th350's.

As for the eating of the carrier bearing do tell.
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Old 09-21-2019, 02:00 PM   #11
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Re: Driveshaft length question

As it left the factory the 1/2 ton coil spring suspension has two solid shafts ( i.e. no sliding spline). When the axle moves up and down the drive shaft has to move back and forth. The yoke on the output of the transmission slides to accommodate this movement. The factory carrier bearing's rubber bushing holds the bearing in position vertically, side to side but allows movement fore and aft as needed. The aftermarket bearings I have seen seem to allow much less movement fore and aft than any factory carrier I have seen. Admittedly I have not found a NOS carrier to inspect.
When you lower the truck the range of movement of the shaft is shifted forward from the factory range of movement. The factory bushing sometimes will accommodate the new range of movement. At other times it fails from operating at the limit of its forward range of movement. And of course it's old. When the factory bushings go bad is is usually noticed as a vibration in the truck as the rubber is no longer able to hold the bearing in vertically and/or side to side, not a catastrophic failure of the mount.
Replacement carriers are designed to do the job as cheap as possible and no GM specifications are used. Thus the reduced amount of travel that they seem to allow. (And don't get me started on the "Engineered to Fail" traits of modern day rubber products. Like rubber bands).
Due to tolerance stacking and how much the truck has been lowered sometimes the replacement carrier will survive. The trick of making sure the new carrier is as far forward as possible will work on some trucks. On other trucks the shaft is too far forward and when the carrier is bolted down the bushing is preloaded at or close to its limit of travel. When the limit is reached the carrier mount has to flex and eventually it breaks.
The same thing will happen if you use a aftermarket billet carrier with the stock 1/2 ton coil spring driveshaft. The billet carriers have no provision to allow fore and aft movement and they require a slider in the rear shaft.
On 3/4 ton and leaf spring trucks the carrier bearing is different and the rear driveshaft has a slider just behind the carrier. The carrier holds the front shaft in one place and the fore and aft movement is taken up by the slider in the rear shaft. There is very little movement of the yoke at the output of the transmission.
The photo shows the factory driveshaft I removed and the new one with a billet carrier and the slider in the rear shaft.
I hope this helps.
Attached Images
  
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The WMB http://67-72chevytrucks.com/vboard/s...d.php?t=698377
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Old 09-21-2019, 03:05 PM   #12
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Re: Driveshaft length question

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Originally Posted by 72LB View Post
I have a 69 shortbed 350/th400 combo and am on my third carrier bearing. I only have a 2.5/4 drop on my truck, does anyone have the measurements for a full length shaft?
if you want to do away with it and have a one piece made all measurement must be made at ride highth measure from center of input on diff. to back of tail shaft then measure to output shaft on transmission thoese 2 measurement should be all you need to make a shaft . that will correct a lot of problems. just my 2 Cents
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Old 09-21-2019, 04:08 PM   #13
AussieinNC
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Re: Driveshaft length question

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Originally Posted by HO455 View Post
As it left the factory the 1/2 ton coil spring suspension has two solid shafts ( i.e. no sliding spline). When the axle moves up and down the drive shaft has to move back and forth. The yoke on the output of the transmission slides to accommodate this movement. The factory carrier bearing's rubber bushing holds the bearing in position vertically, side to side but allows movement fore and aft as needed. The aftermarket bearings I have seen seem to allow much less movement fore and aft than any factory carrier I have seen. Admittedly I have not found a NOS carrier to inspect.
When you lower the truck the range of movement of the shaft is shifted forward from the factory range of movement. The factory bushing sometimes will accommodate the new range of movement. At other times it fails from operating at the limit of its forward range of movement. And of course it's old. When the factory bushings go bad is is usually noticed as a vibration in the truck as the rubber is no longer able to hold the bearing in vertically and/or side to side, not a catastrophic failure of the mount.
Replacement carriers are designed to do the job as cheap as possible and no GM specifications are used. Thus the reduced amount of travel that they seem to allow. (And don't get me started on the "Engineered to Fail" traits of modern day rubber products. Like rubber bands).
Due to tolerance stacking and how much the truck has been lowered sometimes the replacement carrier will survive. The trick of making sure the new carrier is as far forward as possible will work on some trucks. On other trucks the shaft is too far forward and when the carrier is bolted down the bushing is preloaded at or close to its limit of travel. When the limit is reached the carrier mount has to flex and eventually it breaks.
The same thing will happen if you use a aftermarket billet carrier with the stock 1/2 ton coil spring driveshaft. The billet carriers have no provision to allow fore and aft movement and they require a slider in the rear shaft.
On 3/4 ton and leaf spring trucks the carrier bearing is different and the rear driveshaft has a slider just behind the carrier. The carrier holds the front shaft in one place and the fore and aft movement is taken up by the slider in the rear shaft. There is very little movement of the yoke at the output of the transmission.
The photo shows the factory driveshaft I removed and the new one with a billet carrier and the slider in the rear shaft.
I hope this helps.
Such a perfectly presented explanation....agree with 100% of the above...but if you want a one piece shaft, follow the shaft builders measurement process to the letter....
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Old 09-21-2019, 06:35 PM   #14
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Re: Driveshaft length question

HO455 I completely agree with your assessment. I've had my share of 4wd's and never understood why GM didn't do the slip yoke on all two piece shafts. At present I have a full length shaft that only has to be shortened vs building a slip yoke setup, I'm pretty much looking at it from a cost standpoint.

I did solve some of the problems today by installing a shock relocation kit which has made the ride a bit better and it seems more stable of sorts.
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Old 09-21-2019, 10:39 PM   #15
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Re: Driveshaft length question

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HO455 I completely agree with your assessment. I've had my share of 4wd's and never understood why GM didn't do the slip yoke on all two piece shafts. .
They saved up money so they could put natural wood flooring in the back of Suburbans and Panels.
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1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F3 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo
If it breaks I didn't want it in the first place
The WMB http://67-72chevytrucks.com/vboard/s...d.php?t=698377
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Old 09-22-2019, 09:34 AM   #16
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Re: Driveshaft length question

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They saved up money so they could put natural wood flooring in the back of Suburbans and Panels.
I wouldn't doubt that.
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