Correct Driveshaft Length for 700-R4/NP208/14bolt FF

[scrap--metal]

What's the correct length driveshaft between a 700-R4/NP208 and a 14 bolt FF with an 8' bed? Is anybody running this combo and willing to measure u-joint to u-joint for me?

"Frank", my '81 K20, was Frankensteined by the previous owner. As a result, many things aren't quite right. Now I'm trying to figure out the driveshaft.

I had a highway speed vibration and found the rear U-joint needed replacing. I fixed that and my vibration changed, but it's still there. Then, I noticed that there's some slop where the slip yoke slides into the T-case.

I'm pretty sure that my 52.75" driveshaft is too short and it's not engaging the tailshaft bushing like it should. There's about 4 and 1/2" of machined yoke exposed outside of the T-case. All my other GM's have 1" to 2" exposed. If I slide the yoke all the way into the T-case, the driveshaft feels nice and tight.

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[bucket]

Yes, that driveshaft is too short. Ideally it would be about 2.5" longer.

 

[scrap--metal]

Does anyone have a link for the "proper" way to measure driveshaft length? I've heard too many different methods, so I'm looking for a source of truth.

I haven't been able to find a driveshaft locally on Craigslist or Marketplace, so I'm getting ready to have one built.

Can't post in this site's wanted section either because I'm too new...

 

[bucket]

Most driveline shops will want the distance from the u-joint centerline to the end of the output shaft, at ride height.

But you have the benefit of having a working driveshaft already installed. You can simply measure how much farther the yoke should ride in the output housing, then take it to the driveline shop and say "this needs to be "x" longer". They can then re-tube it, replace the u-joints and balance the assembly.

 

[bucket]

Also keep in mind that the output shaft protrudes past the seal by about an inch, so the splines are engaged farther than the seal surface would make it seem.

 

[AuroraGirl]

i dont have a 700r4 but my 80 was frankensteined too, by the PO my gpa lol

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[scrap--metal]

Most driveline shops will want the distance from the u-joint centerline to the end of the output shaft, at ride height.

Thanks for the wisdom, bucket. I have to make some phone calls this week.

 

[scrap--metal]

i dont have a 700r4 but my 80 was frankensteined too, by the PO my gpa lol

AuroraGirl, does that driveshaft fit nice and tight in the transfercase? Or is there some slop?

The driveshaft in your picture looks similar to mine. At least the length of yoke showing out of the transfer case. My K20 came from NW Wisconsin. Maybe all Wisconsin trucks are like this

 

[AuroraGirl]

AuroraGirl, does that driveshaft fit nice and tight in the transfercase? Or is there some slop?

The driveshaft in your picture looks similar to mine. At least the length of yoke showing out of the transfer case. My K20 came from NW Wisconsin. Maybe all Wisconsin trucks are like this

maybe my grandfather touched it at some point :O
My transfer case is slightly too far forward because the crossmember is backward and then where because of that, it pushes the assembly slightly ever so forward. i think the mount is also offset(?) so you can possibly flip for upwards half inch of back or forth. assuming I only have one thing backward, I prob could shave .5 to 1 inch of that open smooth of the slip
iI cannot wiggle the tshaft. well I can, at the u joint a tiny bit not the yoke or the pinion

i also noticed a big uh-oh-cheerios moment reviewing pics of my truck for answering this

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it may just be my failing old eyes, but it looks like the original installed position of the crossmember was either upside down, flipped around on a 85-86, or the original truck had it mounted in frame rails not under,
and that my grandfather not only possibl had things upside down, he had it backward, with no bolt sleeve, with one bolt missing on at least one side....oh grandpa
How much you wanna bet he didnt feel like drilling the offset hole or just square-peg, round-hole was the go to hammer for this lag screw

 

[Grit dog]

maybe my grandfather touched it at some point :O
My transfer case is slightly too far forward because the crossmember is backward and then where because of that, it pushes the assembly slightly ever so forward. i think the mount is also offset(?) so you can possibly flip for upwards half inch of back or forth. assuming I only have one thing backward, I prob could shave .5 to 1 inch of that open smooth of the slip
iI cannot wiggle the tshaft. well I can, at the u joint a tiny bit not the yoke or the pinion

i also noticed a big uh-oh-cheerios moment reviewing pics of my truck for answering this

You must be registered for see images attach

it may just be my failing old eyes, but it looks like the original installed position of the crossmember was either upside down, flipped around on a 85-86, or the original truck had it mounted in frame rails not under,
and that my grandfather not only possibl had things upside down, he had it backward, with no bolt sleeve, with one bolt missing on at least one side....oh grandpa
How much you wanna bet he didnt feel like drilling the offset hole or just square-peg, round-hole was the go to hammer for this lag screw

That's a lag screw holding the x member??
FWIW, the xfer case can not be "too far forward" Unless there's some wizardry there I don't understand...
Motor mounts set the position of the drivetrain first (or most forward) and the connections between the engine trans and xfer case are non adjustable. You cant "move" the transfer case fore and aft. And I totally don't understand the "upside down" comment on the xfer case x member. But based on the many previous discussions you've had about your drivetrain, if the truck goes forward and backwards at 10mph without parts falling out, I'd call it a win for a plow truck that doesn't hit the road.

 

[scrap--metal]

Motor mounts set the position of the drivetrain first (or most forward) and the connections between the engine trans and xfer case are non adjustable. You cant "move" the transfer case fore and aft.

^ Right! Transfer case position is only dependent on the length of the transmission. The variety of transmissions used in these trucks created a need for different length driveshafts. Hence, this thread and my quest for a longer driveshaft.

I'll have to look at my transmission crossmember tonight and make sure it's not in the special Wisconsin position too.

 

[AuroraGirl]

That's a lag screw holding the x member??
FWIW, the xfer case can not be "too far forward" Unless there's some wizardry there I don't understand...
Motor mounts set the position of the drivetrain first (or most forward) and the connections between the engine trans and xfer case are non adjustable. You cant "move" the transfer case fore and aft. And I totally don't understand the "upside down" comment on the xfer case x member. But based on the many previous discussions you've had about your drivetrain, if the truck goes forward and backwards at 10mph without parts falling out, I'd call it a win for a plow truck that doesn't hit the road.

Motor mounts are noticeably yikes and pushed forwar and slightly twisty but that was me The push forward I suspect to be my gpa. The t case mount is also broken, which was me, but it I bet the whole assembly was dropped in in a real tight bind. And no lag screws I was saying he is a hammer and all obstacles he doesn’t have to worry about later are lag screws.
So when he put this in the truck he knew he wasn’t going to touch it again. He was right.

 

[AuroraGirl]

That's a lag screw holding the x member??
FWIW, the xfer case can not be "too far forward" Unless there's some wizardry there I don't understand...
Motor mounts set the position of the drivetrain first (or most forward) and the connections between the engine trans and xfer case are non adjustable. You cant "move" the transfer case fore and aft. And I totally don't understand the "upside down" comment on the xfer case x member. But based on the many previous discussions you've had about your drivetrain, if the truck goes forward and backwards at 10mph without parts falling out, I'd call it a win for a plow truck that doesn't hit the road.

I said or as in that’s the only way to explain the evidence of previously been bolted to the outer holes. See how it is bent up a slightly but there is a rust line/scuffs/a mark that appears it was up against something or may e had a fastener there.

Upside down wouldn’t make sense I think unless some freakish strange mount or adapter went into the little valley that would be there in center but inside the frame rail sure is a possibility

 

[AuroraGirl]

In all, a worn set of mounts and cups mated to a trans and t case the truck doesn’t have bolt holes for and has a crossmember flipped around That MAY have a slight offset backwards rather than correctly or at least the staggered for the bolts is something that very much sounds like a good way to put a bind or improper pressures on a drivetrain with its own size tolerances on a truck with a frame that his it’s own tolerances and was used heavily for 25 years snd had an oversized plow of 10feet from a county dump truck on a homemade attachment a frame on that plow.. with reinforcing spans from the front of the frame to the t case area.. and more than 2 engines, one being a high output race engine for a short time(lots of power to twist and flex things..? Means any number of things could add up to clearances that don’t even have to be half an inch to cause issues with mounting and positioning.

That included the possibility of the too far forward, because I can show you the cups but my guess was pushed the cups the mount bolts through on the engine, but the metal stressed or back and forth twisty twisty to unbind. Then while I used it, it had play and the old mount for t case became the next solid piunt and looks like it had some twist break the edge because it’s not broken from up and down or rubber or the part that retains the rubber. It’s the part where it stands off the rubber and bolts to the adapter with i believe studs on the mount.

 

[Grit dog]

Upside down wouldn’t make sense I think unless ....

I agree, but you've referred to it as being installed or possibly being upside down multiple times...
My point is, it's not nor could it be.