TH400 fixed yoke to slip yoke

[bucket]

So I just swapped a short tail th400 into my C10, the trans had a fixed yoke. I was replacing a th350 long tail. I wanted to keep the stock shaft and swap on a larger slip yoke and remove the fixed yoke that came on the th400. I remember reading somewhere that this swap was as simple as unbolting the yoke and sliding in the new slip yoke. So that's what I did.

Only problem is that the yoke doesn't go very far into the trans before it bottoms out. With the shaft installed (which was very tricky because the lack of travel) I only have about 1/2 inch of travel before it bottoms out. I "think" it will be OK like this, but time will tell.

Has anyone else actually done this swap? Were there any short tail th400's that came factory with a slip yoke? If so, how long was the yoke? The yoke I'm using is much longer than the smaller th350 yoke.

 

[HotRodPC]

Sounds to me the only issue you might be having is that the driveshaft might be an inch or so to long. If it were me, I'd sure like to have more than 1/2 inch. However, it is most likely to have axle drop than axle lift, so you should be OK. Is this in the shortbed truck? I might have a driveshaft that originally came out of a shortbed that was V6/TH400. Might be just what you're looking for.

Doh, on second thought. No it's not. Those were slip yokes, but they were longer output shafts. You're obviously working with a 4in tailshaft.

 

[bucket]

Well that's the thing, this th400 is a little shorter than the long tail th350 that I took out. The driveshaft was actually a little too short because of this, but my large slip yoke I had to install was longer than the th350 yoke, so I was hoping it would make up the difference. When this shaft is installed now, I would call the engagement just adequate, certainly not excessive. There is not enough room for excessive engagement.

So were there any short 4in tail th400's that had a slip yoke or were they all fixed?

 

[SlickGTP]

My short tail has a slip yoke even though it has the two piece driveshaft. Don't remember how long the yoke was though. Might be able to find a picture for estimating purposes though.

 

[SlickGTP]

Well, I can't find any photos to compare... I was sure I had some, but they've seemed to have dissapeared.

 

[bucket]

Well, I can't find any photos to compare... I was sure I had some, but they've seemed to have dissapeared.

Could you do me a favor and measure the distance between your output seal and the "ears" of the yoke? Basically, how much of the machined part is sticking out. I have several inches.

 

[HotRodPC]

All the 3/4 tons with Th400's I seen were fixed yoke but that was in the 80's after GM went back to Th400 in all the 3/4 tons whether it has a small block or big block. The earlier trucks only got a Th400 if they had a big block as far as I can tell. It was up till 77 or so that 3/4 ton small block trucks got Th350's.

 

[SlickGTP]

From the rear seal to the outer edge of the ear is about 5.5" give or take a little bit.

 

[1984dually]

Spline length is the problem between fixed and slip yoke transmissions Slip yoke will only go in too the ledge containing the o ring. not far enough to work with a slip yoke. Not enough engagement with the rear bushing either. Fix is to machine the inside of the slip yoke to go over the o'ring there is a conversion yoke available for just this conversion. Beats the cost of having the output shaft changed.

 

[bucket]

From the rear seal to the outer edge of the ear is about 5.5" give or take a little bit.

Thanks, my length is pretty close to factory I guess.

Spline length is the problem between fixed and slip yoke transmissions Slip yoke will only go in too the ledge containing the o ring. not far enough to work with a slip yoke. Not enough engagement with the rear bushing either. Fix is to machine the inside of the slip yoke to go over the o'ring there is a conversion yoke available for just this conversion. Beats the cost of having the output shaft changed.

So my current setup will probably cause me issues? Do you know where I can get the conversion yoke? I don't plan on keeping this trans in the truck, but at the same time, I don't want to ruin anything.

 

[SlickGTP]

Do you have a one piece or two piece drive shaft?

My slip yoke is paired with a two piece which effectively makes it a stationary slip yoke...

 

[bucket]

It's a one piece shaft because it's a shortbed, so my slip yoke does actually move some. I have about 1/2 inch of travel before it bottoms in the trans, which I'm fairly confident is enough wiggle room for me.

 

[HotRodPC]

I think you'll be OK especially since it's temp use. Your rear axle is more likely to drop than lift. I'd more concerned with it being to short and when the axle drops the driveshaft drop out. 1/2 inch isn't much but should be enough if you're not off-roading.

 

[1984dually]

Denny's driveshafts has conversion yokes available in 1310/30/50 joint sizes. Sure others do to. Bit pricy but not that much more than a normal yoke. Can also be used in a 4L80E with bolt on yoke.

http://www.dennysdriveshaft.com/c75_general_motors.html

 

[bucket]

Looks like with that yoke I would need the shaft to be a little longer.

So from what I'm gathering, right now I have optimal spline engagement, the yoke just plain has no contact with the support bushing. Correct?