Wrong yoke for my TH400? Counterbore?

Hey guys,

After painstakingly measuring the driveshaft length for the Turbo 400 I'm putting in my nova, I finally concluded that I currently have 3/8" play and needed an additional 5/8" to get me to my 1" goal. I took it to the fab shop today to have it cut down and, luckily, the tech noticed that I didn't have as much of the yoke going into the tranny as I needed based upon my markings. This is my first 400, so I wasn't sure what to expect - but I'll bet that only 2/3 of the machined yoke surface actually fits into the tranny. Basically, he told me that my 600+ HP would twist the portion of the yoke that was in the tranny off, and that I needed to look for a "counterbore" yoke for my tranny.

I'm going to the local race shop that I work with tomorrow to see what might be going on, but thought I would run it past the board beforehand. I did a search but didn't find anything that seemed to relate to this topic, so I apologize if I'm asking for something that's been answered elsewhere.

Thanks...

Hobo

the 1350 yokes do not have any counter bore spline up to the end of the yokes is this what you mean? the spline is what makes it strong not the counter bore if you have more then a 1 1/2 hookup on the spline you should be good

When I built my roadster pickup driveshaft, I used a turbo 350 automatic tranny and rear yoke, to get the correct length of driveshaft. I slid the yoke into the tranny tailstock untill about an inch of it was still exposed, then built my driveshaft to suit. You can imagine my dismay, when a while later I was installing the driveshaft, which means that you must slide the yoke all the way into the tranny to let the rear universal move into place in the rear axle yoke---and the darned yoke wouldn't slide in that last inch!!! That one was a real head scratcher, untill Denny or Pat (I can't remember now which it was) on this forum come up with the suggestion that since it was a used front yoke, the internals splines might have gotten twisted during a "hard launch", which would prevent it from sliding in all the ways. So---I got out my trusty mag lite and looked up inside, and sure enough!!! About 1" from the bottom there was a definate "tweak" in the spline. I ran off to the auto parts store and bought a new front yoke, it slid all the way into place, and everything worked fine. I thought for about an hour that I had cut my driveshaft to the wrong length.---Brian

Probably not your problem, but one to be aware of.

I have a dummy T-400 in my 31 on 32 rails roadster project.

I couldn't get the yoke to slide on more than a half inch or so.

Got a jack under the trans pan - with board to spread the load and floor stands - and removed the tailshaft housing.
It's a short one on the big Buicks.

Found the splines loaded with grit and dry mud etc.
The trans probably sat out in the weather awhile before I got it.

Wire brushing the splines clean allowed the yoke to slide on and bottom out.

Next stop, driveshaft shop to cut 1" out of the shaft.

Pat - after I cut the driveshaft for 1" play, I would have about 1 to 1.5" of yoke actually hitting the splines of the tranny if the suspension were to bottom out...

Brian - I did see that thread, but my yoke is a new Spicer unit that I just purchased. Regardless, I'll take a look.

I'll still check with the race shop tomorrow. It seems like the yoke should be sliding in much further than it is, but I'm new to the auto-trans stuff - I guess I'm a hardcore "third pedal" guy at heart...

Guys,

I just spoke with Denny at Denny's Driveshafts, and I have to say that he was a TON of help. We're both betting that my TH400 is a threaded "truck" or "bolt-on yoke" style, which I will verify tonight. If that's the case, there is indeed a counterbore inside and a counterbored slip yoke is made to address the issue. All I need to do is verify my u-joint cap measurements tonight along with the driveshaft and order the correct yoke.

Thanks everyone - I guess the learning never stops...

Hobo

Okay all - one more question. Before I purchase the $100 "counterbore" yoke, I decided to pull the tail off my tranny and see what was causing the yoke to stop it's travel. Turns out that the threaded shafts have a metal shoulder about 1/3" wide with an o-ring that is stopping the yoke from going the full length onto the shaft. I'm thinking about pulling out my p-grinder and smoothing out the metal shoulder, which would allow my yoke to slip in all the way and I likely wouldn't need to have my driveshaft shortened because of the extra slip it would give me. (It would also save me $100).

Anyone know of a reason why I shouldn't do this, or problems it may cause? I wouldn't think that by smoothing this out all around the shaft I would cause any balance issues.

Thanks...

Hobo

Hobo, think about the torque that twists the splines on the yoke....now think about your grinder leaving nicks or stress risers in the shaft....what you would be trying to do is to make a lathe part using a hand held grinder!! What if you screw up and the shaft breaks? Is that a $100 error? What if the shaft breaks and the yoke goes sideways and takes out the transmission tailhousing/case? That may be at least $3 alone..... If you're asking, my advice is to pony up the $100 and do it right.

mike in tucson

That spacer is a good thing-keeps your yoke from getting harmonics that could lead to vibration. Just spring for a different yoke summit has them also. I went through the same thing with mine-good luck.

Okay guys - I'm convinced. That's why I'm a member of this board.

Thanks everyone...