TH400 ID Please

The metal tag on the right side reads (77-PA 16201) I believe it came from a 67 Le Sabre. The OD of the trans yoke is smaller in diameter than a standard 400 but the ID and splines (31) are correct. I'm under the assumption it's an early year 400, but is it a SP or VP trans? It only has (1) one prong connector on the side and a vac mod as well. Any correct info is appreciated. Thanks...Tim

 

You have a 77 PA (means 1977 Pontiac trans).
There are two different size yokes for a 400, pre-70 had a larger diameter than 70-up. Also, don't assume it is a 400. If you have a TH350 yoke and it fits in the tailshaft, you have a TH375 trans which was a compromise between TH400 and TH350. Also, make sure it is a shortshaft trans.

 

Hmmmm

Hi George. As a driveline repair specialist of thirteen years i've built my share of all types of shafts. I mean ALL types of cars and trucks. This is not a 27 spline output as the 350 / 375 has. Its a short tail 400 (31 spline) but with an unusual size O.D. which is why i'm trying to get it identified correctly. I'm well aware of the several different 400 lengths (thats another story) but the O.D.'s are pretty standard and i've not seen this style yoke except on VERY early 400's. You may be absolutly right about it being a 77 Pontiac trans. The guy who sold me the car had the trans rebuilt by a friend of his at a tranny shop. I wonder if he swapped tranny cases and put the original rear output back on to match the original trans yoke that was already on the driveshaft. Now that would make sence. Thanks for any help gents...Tim

 

I learned the hard way about the two different output shaft diameters on the TH400. I was stripping a 68 GS w/TH400 and was removing the engine and trans to give to some fellow Buick nuts. I grabbed one of my handy TH400 yokes that I removed from a 73-77 GP TH400. Slid it into the trans and began pulling the combo. Was getting dark and was working with lights. I suddenly hear this running fluid sound and out of the trans t-shaft, trans fluid was running on the ground. I was pissed. Turns out the earlier TH400 yokes are about 1/8-3/16" diameter larger than the later versions. So, I have now purchased one of the universal t-shaft slip yoke plugs for this never to happen again. Best $8 I have ever spent!

Sounds like your assumption of the earlier t-shaft being swapped into the newer case is correct!

Did not know your background so that is why I stated the TH375 being made by GM in those days!

 

Thanks George. I do believe there has been some swapping going on! :Brow:
And Larry a 301? a measly 301? ou: Engineers! :beer to you both!

 

To the best of my knowledge, the small diameter housing was only used by Buick and last used on the 70 Riviera. You can usually identify it by 3 aluminum tabs that are cast on the side of the housing.
The larger diameter ones came out on the 67 short tail 400's typically on the GS400, GTO, 442 and ss396. Usually there is a letter "C" cast into the side of the housing. Sometime around 73, extra webbing was added to the mount area for added strength. In the mid to late 70's, these housing used metric threads for the mounting bolts.
Sounds like the trans you have is a TH375, which is basically a TH400 with less clutches in forward and direct. Not a bad unit, and it can be upgraded by using the earlier pistons to add more clutches.

Mark

 

we're good now

Hey Mark. To my knowledge the TH375 uses the 1.500 dia. 27 spline yoke as the TH350 does. The TH400 being 31 spline with a 1.886 diameter. What prompted my question is that my trans yoke has a 31 spline 1.686 O.D. Which is essentially smaller than what i'm used to seeing in a 400 trans. Except I was thinking the thinner wall yokes were early and the thicker wall yokes came after GM realized they needed to beef them up a bit. Seeing how this trans came out of a 67 Le Sabre with a 71 350 I wasn't sure what I exactly had.

 

The only ones that I am aware of that had the smaller output shaft were on chevys. Got a trans from a 73 Kingswood wagon once that had a long tail, but used the TH350 yoke. Wound up switching the case to a BOP and used it in a 71 LeSabre. Fit perfect with no driveshaft modifications.
The short tail TH 375's that I have seen over the years all had the larger output shaft and housing.
There are actually 3 tailshaft lenghts. Short ones used in the "A" body cars, medium (usually referred to as long) for "B,C" and long for some Cadillac.
I know the trans in the 67 LeSabre is the short one. However, you never know what could have been put in. Have never seen the smaller output shaft diameter you mentioned on a short shaft trans, but who knows.
The OD on the 64-68 LeSabre/Wildcat/Electra/Riviera should be the 1.686 that you mentioned with 31 splines and be a short tailshaft. Sounds like all they did was take a later trans (short tail TH375) and put the 67 housing on it so the driveshaft yoke didn't have to be changed. Have done tha myself many times too.
Mark

 

Yeah, I know what you mean about the three different output lengths. I was a driveline repair specialist for 13 yrs. Figure on average five shafts a day, new or rebuilt, MG small to Mack truck big, six days a week. I saw lots of different style yokes from all different mfgs., BUT because we didn't work on the actual vehicles, I had to ask the customer what kind of trans they were running. Just to be sure they even had the right yoke for their application. Thats why I thought the 375 was only a med length tail utilizing the TH350 yoke. You wouldn't believe how many rusted "I got it from the junkyard" shafts people brought in to be shortened for their "race car". ou: Bent, dented, wrong series u-joint, too small a diameter, just plain old crap. The biggest problem with the TH400 would be the "truck trans" outputs. (TCI trannys are good for this) Even the shortest 400 yoke still has to be lathed down shorter and the inside back-cut so it will slide into the trans far enough to pass the rear end yoke and then slide back out the needed 3/4 in. from the trans. When I don't know I ask, but when I know I'm right LOOK OUT! :laugh: :laugh: :TU: