th400 fixed yoke or slip yoke?

[Greasey Harley]

I'm getting read to swap my '68 c10 to an automatic transmission.
The transmission builder wants to know if I want a slip yoke or fixed yoke on the th400 he's building for me.

keep the 2 piece driveline or install a 1 piece
it'a a short bed, how were they setup from the factory?
should I keep the 2 piece or convert it to 1 piece?
fixed yoke or slip yoke?

My thought is; install the fixed yoke and use the existing 2 piece drive shaft,
it's already there and is in good condition.
Just shorten the front shaft.
is this the best route?
The truck is static dropped 3.5/5, the current setup works well. it's just a cruiser, but hauls the occasional motorcycle or load of mulch, if that matters

[Joyridin]

If I had the whole set-up from a 2 piece, I would reuse it. The carrier bearing is a bit of a pain at times, but it is hard to beat "free" when comparing the price of a 1 piece. I have had my driveshaft in my car modified numerous times. It is $100. It is too short to mess with balancing, so that is a deal versus a new shaft.

I have a fixed yoke on my 1 piece...at least that is what the driveshaft shop told me. It is about 1" - 1.5" shorter than a slip yoke. I moved the rear up and down in its max and min travel area and the yoke barely moved in and out. As long as you have it in far enough and the travel isn't pulling it out, then use whatever, although with a 2 piece, it isn't an issue.

[kwmech]

With a 2 piece it will be a slip. If going with a 1 piece get the fixed. Short bed 1 piece, long bed 2 piece

[cadillac_al]

I would vote to keep the 2 piece if it came that way and is working fine with your drop. If it isn't working fine with your drop then it is a good time to go to a one piece.

[Ironangel]

You want a slip yoke behind the 400TH and keep the two piece shaft! I just installed a 400TH in my big block truck and the only change I had to make was shortening the front shaft 1" and having the drive shaft shop weld a 1350 yoke to the end to match the 1350 slip yoke. I bought the clam shell carrier bearing and let him install it when he assembled and balanced the whole two shaft assembly. I beat the hell out of that shaft assembly with a 439ci BB that lives at 7000 rpm. A two piece shaft is stronger and will turn more rpm than a single shaft. The only time you want a fixed yoke on the trans is because there is a slip joint on the other end of that shaft. The two piece shaft assembly behind an automatic trans should have a slip yoke at the trans output shaft and a solid shaft through the carrier bearing. A bolt on Yoke with splines will bolt to the end of that (primary) shaft. The final or (secondary) shaft may or may not be a solid shaft. It depends on what carrier bearing was used, there were two types. One is a clam shell design that hangs on a stem, the other is a pillow block design that is U shaped and bolts on each side. Which carrier bearing does your truck have? The clam shell carrier bearings are designed to flex fore and aft as the rear axle articulates up and down. The pillow block carrier bearings had splined slip joints on the second yoke. That allowed the second shaft to move with the axle and slip on the splines without stressing the carrier bearing. DO NOT use a billet carrier bearing, or a bearing mounted in a rigid polyurethane bearing mount unless you use a slip jointed secondary shaft with it! Pinion bearings and transmission thrust bearings will not live without slip yokes! With your drop and going from a manual to an automatic trans, your going to need to keep the crank and trans centerline level and as close to inline with the center of the carrier bearing as possible! That may require shimming the trans tail housing up or down to keep that primary shaft flat and inline with the carrier bearing. You need to avoid any angle where the slip yoke meets the primary shaft! Your pinion should be parallel or as close as possible to parallel with the primary shaft. That way the final two u-joints will cancel each other and your world will be in harmony and balance... Similar to the image below, disregard the second u-joint...

[Ironangel]

Just to be clear and that we all understand the difference between a fixed and a slip yoke... First pic is a slip yoke, it simply slides on the output shaft splines. The second pic is a fixed yoke that is held in place with a washer and bolt, it will not slide.

[Greasey Harley]

Originally Posted by Ironangel

...only change I had to make was shortening the front shaft 1" and having the drive shaft shop weld a 1350 yoke to the end to match the 1350 slip yoke. I bought the clam shell carrier bearing...

... The only time you want a fixed yoke on the trans is because there is a slip joint on the other end of that shaft. The two piece shaft assembly behind an automatic trans should have a slip yoke at the trans output shaft and a solid shaft through the carrier bearing. A bolt on Yoke with splines will bolt to the end of that (primary) shaft. The final or (secondary) shaft may or may not be a solid shaft. It depends on what carrier bearing was used, there were two types. One is a clam shell design that hangs on a stem, the other is a pillow block design that is U shaped and bolts on each side. Which carrier bearing does your truck have?
The clam shell carrier bearings are designed to flex fore and aft as the rear axle articulates up and down. The pillow block carrier bearings had splined slip joints on the second yoke. That allowed the second shaft to move with the axle and slip on the splines without stressing the carrier bearing.
...your going to need to keep the crank and trans centerline...

The current transmission (SM465) has a fixed yoke and the pillow block style carrier bearing. slip yoke on the rear drive line.
I know how to set up driveline angles & phasing, not a problem, just trying to figure out the correct configuration
Since I have the slip joint on the rear shaft it seems reasonable to use the fixed yoke on the trans...I think?
It sounds like that's not how they were from the factory though?
I'd kinda like to replicate the factory setup if possible. On the other hand, easy is good.

Its the same price for either trans yoke, I just need to tell the transmission builder which one I want.

The factory setup had a slip yoke on front and rear driveshaft?

[Joyridin]

Originally Posted by Ironangel

Just to be clear and that we all understand the difference between a fixed and a slip yoke... First pic is a slip yoke, it simply slides on the output shaft splines. The second pic is a fixed yoke that is held in place with a washer and bolt, it will not slide.

That is not what my driveshaft shop stated, but your description would seem obvious.

The TH400 yoke on my truck was very short. Probably 1.5" shorter than one in a car. He used the term "fixed yoke" because with the 2 piece driveshaft, the yoke would generally not move so they used a very short version. With a 1 piece driveshaft, the yoke was a lot longer to allow for extended travel.

I put the short version back in and it works fine. If my truck sat at normal height with a lot of suspension travel, that short yoke might be an issue with a 1 piece shaft.

[Ironangel]

Originally Posted by Greasey Harley

The current transmission (SM465) has a fixed yoke and the pillow block style carrier bearing. slip yoke on the rear drive line.
I know how to set up driveline angles & phasing, not a problem, just trying to figure out the correct configuration
Since I have the slip joint on the rear shaft it seems reasonable to use the fixed yoke on the trans...I think?
It sounds like that's not how they were from the factory though?
I'd kinda like to replicate the factory setup if possible. On the other hand, easy is good.

Its the same price for either trans yoke, I just need to tell the transmission builder which one I want.

The factory setup had a slip yoke on front and rear driveshaft?

The SM465 2WD transmissions used a fixed yoke on the output shaft. That required a slip joint on the final drive shaft. The pillow block carrier is the heavier duty carrier bearing. This was indeed a factory set up for the heavier duty C/20's and C/30's. My 71 C/30 has that exact set up except with a NP435... Many of the 1-tons with 400THM transmissions had bolt on yokes behind drive line E-brake units like this one shown.

[Greasey Harley]

so here's a pic of my current setup.
transmission has a fixed yoke to a solid shaft that goes through the carrier bearing.
rear shaft has a slip yoke up front then down to the rear end.

Obviously, it is simple to shorten the front shaft and install a fixed yoke to the trans.

its not that much harder to delete the entire thing and have a 1 piece driveshaft made with a slip yoke to the trans.

I guess the only thing I really want to know is;
Which way would it have came if it had a TH400 from the factory? (it's a 6' bed)