MDchanic

So this crate I'm working on had a "little" driveshaft problem - like someone cut it off 1½" short and had his kindergartener weld the end back on - so I've procured a new driveshaft for it (Thanks, John!), and was in the process of putting my slip-yoke on it, when I ran into trouble.

Now, to go back in time a bit, last month I ran into the previous owner, and I was talking to him about how I was planning on getting the driveshaft balanced to get rid of that nasty vibration ("I don't remember no vibration - she was smooth as glass!"), and he told me about how one of the reasons he got it cheap from the owner before him was that transmission fluid was pouring out of the back of the tranny. He said he found that the leak was coming from the center of the yoke, so he JB-Welded it, and didn't have any more problems. Sure enough, the back of that yoke has enough JB-Weld on it to keep the Titanic afloat.

Anyhoo, today I pound the yoke off the old driveshaft and crawl under the car to be sure it fits all the way in (I've heard of TH-400 yokes that are splined differently and don't go in all the way, and the yoke not going in would have been a plausible reason for some idiot to shorten the driveshaft). I hit the usual air lock as I push it in, so I give it a good hard shove, and it slides home, all the way to the bottom, just like it should. When I remove it, I discover that when I pushed it, I exceeded the engineering parameters of the JB-Weld, and the little vent tube cracked through the JB-Weld to release the air.

So now I'm in a quandary.

Do I re-JB-Weld the vent hole up (or use silicone or braze the darn thing),
-OR- do I fix the root problem that caused the leak, which, after reviewing the interwebs and my CSM, seems to be a bad O-ring around the output shaft, that slides into the smoothly machined portion of the yoke above the splined area and keeps oil out of the splines (which should be greased)?

So, the big question is: How easy or hard is it to change that O-ring?
It looks like if I remove the tailpiece, I can just roll the old O-ring off an a new one on.
How hard is it to slide off the tailpiece on a '73 B-body?
Do I just remove the four nuts and the speedo drive, support the trans., and tap it off? Is there clearance? Will disaster strike?
Has anyone actually done this before?

I await your learned responses.

- Eric

69442C

Holy Crap! I just went though this last week. Finally backed the resto 69 442 project out of the garage after 5 years and move it up and down the driveway a little (despite the fact that the car isn't running right). Parked it in the driveway and noticed a decent size puddle of trans fluid under it. I'm thinking "Now what the hell is going on with this car" ? (I swear it's possessed) I climbed under it and saw just what you described in that oil was coming out of that little hole in the center of the yoke. Of course the front ujoint did a nice job of throwing oil around quite a bit so that needed to be dealth with too. I pulled the shaft, removed the yoke, welded the hole and put it back together. But now from what you wrote, I wonder if that was the correct thing to do.

To answer your question, yes, just remove those bolts, unbolt the trans mount, disconnect the speedo cable and the tail housing will come off. I'm pretty sure there is an o-ring that seals that tail piece to the main body of the trans.

TripDeuces

My TH400 yoke had that same little hole. I just welded it up

dhoff

Is the hole there to let the air out when the yoke is pushed on to the shaft when the rear end is hitting a bump? Or why would they vent the yoke?

MDchanic

Yes.

The output shaft splines are supposed to be lubricated by grease, not ATF, so there's an O-ring on the output shaft that fits into the yoke to keep oil out and grease in.

So long as that O-ring is intact, there will be no oil in the inside of the yoke, but that also means that the inside area of the yoke is sealed up pretty well, and any fast movements of the rear end can compress the air inside the yoke, possibly causing the yoke to rap against the output shaft, damaging the transmission, even though it never actually touches the end of the shaft.

Because the inside of the yoke is dry, it can have a vent hole to prevent this from happening, but once the O-ring wears out, and oil finds it way in, it's only a matter of time before that oil comes out through the hole.

Apparently, replacement yokes are made missing one spline, which acts as an air vent back through the transmission, without allowing a significant amount of oil to get back in (if oil got in, and the yoke were hammered in on a bump, the non-compressible oil could definitely cause problems).

Anyhow, now we've got two people who welded theirs up.
Any other takers?

- Eric

69442C

My yoke was missing one of the splines. Also, my trans was a fresh rebuild by a very reputable shop. I looked in the 1969 CSM for the driveshaft and the trans and I can't find any reference to the o-ring on the output shaft. I'll have to look a little closer and I may question the trans shop to get their input.

MDchanic

Mine makes no reference either, but you can see it in the cutaway view, and the yoke has that first inch and a half or so of smooth bore for it to ride in before the splines start.

- Eric

69442C

I stopped by the transmission shop that did my trans and asked about this issue. This shop says they always seal the hole in the yoke with a weld if they have the car present and are doing the R&R on the trans and thus the driveshaft is there. But they rarely see this anymore. When they build a TH400, or other GM transmissions of similar vintage, they do not install that o-ring on the output shaft, mainly because if a yoke would ever be used that did not have the hole and the o-ring is present, there will be an issue because the yoke can't breathe when it slides in and out. They do a fair number of trans rebuilds where the trans is brought in by the customer who is doing their own R&R. They said the trans has a vent on the top and that the yoke will not cause the trans to be pressurized when it slides in and out because of this vent. They said most of the replacement yokes they see do not have this hole in it and that the trans fluid will work just fine lubricating the splines.

It sounds as if the o-ring on the output shaft with a vented yoke is fine as is no o-ring and a non vented yoke. This shop prefers to not use the o-ring as they feel this is a better approach for them especially in situations where they do not have the car in their posession. This shop also does a lot of trans work for race teams and I'm sure they custom build driveshafts and likely do not have vented yokes.

My trans is now a non vented yoke since I welded the hole closed and I now know that the output shaft does not have the o-ring so I am fine.

If you keep the vented yoke, you'll just need to be sure the o-ring is present and in good condition or else there will be a leak. and it could become a maintenance issue down the road.

Brian

MDchanic

Interesting.

Of course, it's the same amount of work to replace the old O-ring as it is to remove it completely...

We'll see what it looks like when I get it apart.

Of course, the vent tube on mine has something wrong with it - it's about an inch and a half long tube, a bit like a piec of brake line, with slightly flared ends, but it's not attached to anything, and is free to slide in and out. Also, the outer end doesn't have an obvious hole in it - it looks crushed shut.

Anyone know what this thing is supposed to look like?

- Eric

J-(Chicago)

Man, I've never had any of these problems before. Haha. All of my yokes have 1 flat spot in the splines, no holes in the back, and no o rings on the tail shaft.



The o ring on the output and a counter bored slip yoke is a truck thing as far as I can remember.
Hell, the th375 was only a buick and chevy truck thing I thought.
My 72 delta had a th400, and the 77 98 had a th400.

Where do you guys find this junk?
Did they sneak th375 in olds from 73-76 somewhere???

MDchanic

TH-375 came standard on all full-size cars about '71-'75 with small blocks,
TH-400 came on big blocks.

- Eric

J-(Chicago)

Ahh... thanks for the clarification. I have one of those th375's from a buick, but it seems to have a 400 front case with a th350 output shaft. what an odd duck.

Do you know if it has the same amount of clutches and pistons as the 400?

MDchanic

That's what a TH-375 is: a TH-400 with a different tail housing and tailshaft, to take a TH-350 yoke.

There were a huge number of variations in numbers of clutches, etc. in the TH-400s, and though I don't know specifically, I believe that the 375 had the same internals as a lighter-duty 400 (like the kind used with 2bbl cars). I really don't know what the advantage was to GM of making a transmission to take a TH-350 yoke, especially when the driveshaft itself was the same for all engines in those B-bodies, but they did it.
On the good side, it means that all you need to swap in a TH-400 is the transmission and a slip yoke - no BSing around with driveshaft lengths and crossmembers.

- Eric