Why did my drive shaft just fall out of the transmission?

[MARKDTN]

I would almost guess that the PO had a rear u-joint failure. He went to the junkyard and got a rear driveshaft out of a 3/4 or 1 ton that is shorter and put it in. I think your rear shaft is too short.

[centuryhouse]

Center of wheel to center of wheel is 127", which is also what is listed on my SPID.

The front shaft (runs from transmission to carrier bearing) is around 32" (shaft itself) and add another inch or so for the u-joint part, or another 6-8" for the splined part.


I don't think the front shaft can be the wrong length; if it were any longer it would be too long for the carrier bearing.

The carrier bearing and cross-member that holds it appear to be correct - nothing looks rigged, and it all appears to be in the original position.

The rear shaft appears to be the same length as others have, based on comments above.

That leaves me thinking the yoke might be the problem. Could it be a few inches too short? Connected, it only covers less than an inch of the splines of the front shaft. How long are the yokes on your trucks, if you have this setup?

[BossHogg69]

I wouldn't feel comfortable with anything less that 2" of engagement. You risk either the driveshaft falling out again or breaking the end off the splined part of that intermediate shaft without proper engagement. You should at least take the rear shaft, if not the whole truck to a driveline shop and tell them what is going on so they can find you the right yoke or build you a longer driveshaft. You're lucky that it didn't catch the ground and pogo your truck in the air and really tear stuff up when it fell out. You can measure and figure all you want, but the thing is too short and needs to be fixed.

[centuryhouse]

Can anyone with the 127" wheel base get me a measurement on their slip yoke (think that's what it is called)?

[kajf9491]

Is it possible the PO had an overdrive trans installed (say 700R4) which needs to have the drive shaft shortened (2in I believe), then before sale threw the old tranny back in it leaving the drive shaft the same?

[KEL]

I have a mostly stock 1972 GMC 1/2 ton 127" with a 350 / TH350.
I just went out to the barn an took a quick measurement. Weld
to weld on my front shaft is about 24". My tranny tail is about 6".
I was having trouble because this thing is not up in the air.
I couldn't get to the "yoke" w/ splines which slides into the tranny.
If no one chimes in, I can measure whatever you want on the
weekend - - - if you remind me !

[centuryhouse]

Originally Posted by KEL

I have a mostly stock 1972 GMC 1/2 ton 127" with a 350 / TH350.
I just went out to the barn an took a quick measurement. Weld
to weld on my front shaft is about 24". My tranny tail is about 6".

My 72 has the 350/TH350 also. Front shaft measures around 32" (shaft itself, not splines or u-joint 'ears').

[scimfish]

So I'm looking for information as well. I have a 71 C20 that I changed the rear end in to a 1/2 ton. Can I just find a rear shaft from a similar year 1/2 ton and put it in place of the 3/4 ton shaft?

[alsriv2]

Due to the huge amount of changes and variations that could have been done throughout the years to your truck, it might be best to just run it down to a drive shaft shop to get yours fixed correctly. Each of us probably has some combination that has worked well enough to keep our trucks going, and while a good many of them might be original or close to original, its hard to comment on your particular setup.
I had a 67 GMC 3/4 ton truck years ago that had a weird setup as well as that truck had been used as a trailer hauler and camper hauler for years. Due to various repairs done to the drive line by creative individuals, when it came time to actually get parts to fix my driveshaft failure, I had a nightmare on my hands.

But many here are providing good reference information, so I wish you the best on finding a solution to your problem.

[Baddflash]

Ok my '69 has original 3-spd and driveshafts, 127" wheelbase. The rear shaft is 31.5" from weld to weld or 36.25" from the u-joint's. Your rear shaft is too short.

[Baddflash]

Here's how that section looks on my truck.

[Keith Seymore]

Originally Posted by alsriv2

Due to the huge amount of changes and variations that could have been done throughout the years to your truck, it might be best to just run it down to a drive shaft shop to get yours fixed correctly. Each of us probably has some combination that has worked well enough to keep our trucks going, and while a good many of them might be original or close to original, its hard to comment on your particular setup.
I had a 67 GMC 3/4 ton truck years ago that had a weird setup as well as that truck had been used as a trailer hauler and camper hauler for years. Due to various repairs done to the drive line by creative individuals, when it came time to actually get parts to fix my driveshaft failure, I had a nightmare on my hands.

But many here are providing good reference information, so I wish you the best on finding a solution to your problem.

I agree; a lot of discussion here because there are so many variables.

If it were me I would drag it to a local driveshaft shop and have the appropriate shaft made. After a couple hours and a couple hundred bucks you would be on your way, with confidence that it is right.

K

[centuryhouse]

Originally Posted by Keith Seymore

If it were me I would drag it to a local driveshaft shop and have the appropriate shaft made. After a couple hours and a couple hundred bucks you would be on your way, with confidence that it is right.

K

I understand. I talked to two driveline shops the other day. I'll have to have it towed about 15 miles to do that.

What I'd really like to know is how long is the slip yoke on others' trucks who have the same length rear drive shaft.

That answer might save me a few hundred dollars (money I will not have for at least a month). Or not. But it'd be useful info if I found, for instance, that others yokes were 7" instead of the 3 /12" I have.

[ERASER5]

The slip yoke looks normal. When swapping drive shafts, getting it perfect is very difficult. There are a lot of variables. In my case, my DS is too long by about 3/4". New springs my fix that. Yes. Ride height will change how much the slip joint engagement is. That's the purpose of the slip joint, to slip in and out as the truck takes a bounce or is loaded.

[hamjet]

Originally Posted by Baddflash

Here's how that section looks on my truck.

This is the style I have, which is not a slip joint, front and rear shafts are fixed together by the u-joint..

[centuryhouse]

Originally Posted by ERASER5

The slip yoke looks normal. When swapping drive shafts, getting it perfect is very difficult. There are a lot of variables. In my case, my DS is too long by about 3/4". New springs my fix that. Yes. Ride height will change how much the slip joint engagement is. That's the purpose of the slip joint, to slip in and out as the truck takes a bounce or is loaded.

Good to know.

The thing about this slip yoke is that its body is only 3 1/4" long, and the exposed spline is 5 3/8" long.

If that yoke engages even just 2" of the shaft (which is not enough, IMO) then it only has 1 1/4" of play to 'slip'.

I can't imagine it is supposed to be that way, with 3 1/2" of spline showing all the time.

Which is why i keep asking how long the yoke is for those with a 28"-29" rear drive shaft like mine.

[Matt Cramer]

Originally Posted by kajf9491

Is it possible the PO had an overdrive trans installed (say 700R4) which needs to have the drive shaft shortened (2in I believe), then before sale threw the old tranny back in it leaving the drive shaft the same?

Unlikely. If the overdrive had been installed correctly, you'd shorten the front shaft, not the back.

I'm wondering if the truck is a Longhorn - the longer long bed. And using a regular long bed driveshaft.

[Keith Seymore]

Originally Posted by centuryhouse

That answer might save me a few hundred dollars (money I will not have for at least a month). Or not. But it'd be useful info if I found, for instance, that others yokes were 7" instead of the 3 /12" I have.

I sincerely apologize; I forget that not everyone has extra money for this kind of thing until payday rolls around.

Which is today, by the way. (Yay! )

K

[centuryhouse]

Originally Posted by Matt Cramer

I'm wondering if the truck is a Longhorn - the longer long bed. And using a regular long bed driveshaft.

Somebody mentioned that earlier, and I really thought that was probably it - but it turns out my SPID says 127" (not 133") and when I measure wheel center to wheel center I got 127".


And another poster above indicated his rear shaft was also 28"-29", like mine. What DOES look non-standard is the slip yoke - mine has threading on the end which is not typical on our trucks, and appears to be shorter than standard. I just need someone who has the same length rear shaft to verify the correct length of their yoke.

[geezer#99]

I've only seen the yokes like yours that bolt on to the splined shaft. Like in post 36. You can see the head of the bolt right in there by the ujoint.
Can you take a pic of the end of the splined shaft you have. Possible the bolt is broke off.

[centuryhouse]

Originally Posted by geezer#99

I've only seen the yokes like yours that bolt on to the splined shaft. Like in post 36. You can see the head of the bolt right in there by the ujoint.
Can you take a pic of the end of the splined shaft you have. Possible the bolt is broke off.

The one pictured is a different design, with a bolted yoke (?) instead of a slip yoke. My yoke slips right over the spline:

[geezer#99]

If you unbolt his and slide the yoke off you'll see splines just like yours.
And yours looks like it has a broken headed bolt in the end.

[centuryhouse]

Originally Posted by geezer#99

If you unbolt his and slide the yoke off you'll see splines just like yours.
And yours looks like it has a broken headed bolt in the end.

If a slip yoke is bolted in, it would no longer be a 'slip' yoke because it can't 'slip', right? Which would create stress where there shouldn't be any, right?

And wouldn't the yoke need a hole in it for a bolt to go through?

And no bolt in the end if the splined shaft - a hole with grease in it. Is that where you are saying a bolt should go? Through the yoke from the u-joint area, and into the splined shaft?

[KY_GMC]

Look at my post from 7/29. The length of the yoke from the center of the u-joint to the end of the yoke is around 6".

The threading that you refer to is for a grease cap. Yours is missing.

[centuryhouse]

Originally Posted by KY_GMC

Look at my post from 7/29. The length of the yoke from the center of the u-joint to the end of the yoke is around 6".

The threading that you refer to is for a grease cap. Yours is missing.

Thanks. I saw your post/ My yoke is around 5 1/2 inches (without dust cap). And my rear shaft is also 28" like yours. The difference, is that mine only engages less than 1" of the spline, where yours appears to engage several inches.

I can't figure out what is different about ours.

Could the rear end effect this? It looks like mine is a 12-bolt Chrysler 8.75 - not sure if that was original or not. It seems like the rear end is the only other component that should effect where the parts line up for that rear shaft.