[West Coast GTO]

Do all turbo-hydro 400 transmissions of any given year shift the same and are they interchangeable? For example, does a 71 TH-400 code PX shift the same as a 71 PR code( 400 cid compared to 455 cid)?

[Schurkey]

Yes. They all shift 1--2--3 as speed increases. They all shift more firmly as the load increases. They all have kickdown provisions.

No. There's a million differences in valve body calibration, accumulator differences, clutch-pack stuffing, modulator and governor calibration.

So they're all the same but completely different depending on your perspective.

[Half-Inch Stud]

As for Pontiac TH400s, there are about 5-6 differences in VBs since 1965-66, and about 1-2 VBs since 1970. There will be differences in Separator plates, like under 10.

As for the rest of GM, more differences, but not many.

As for the internal parts, same deal where each part has a variant through the years, and some key parts have significant quality change:

Prefer the AH or A VBs, with the V ) V or V V Separator plates. No skift kit hole changes needed, just use only the 3 checkballs that lay along the centerline.
Flip the 2-3 accume in the VB.

Do the Direct Pack lip middle seal removal, 2nd Center support ring removal, and use an 0-80 screw-nut to block the tiny Reverse Bleed hole in the separator plate.

Inspect the Front Pump gearset condition, use a 121, or 893 pump assy, shim the baby blue Regulator Spring, or use an Orange Reg Spring.

Direct Pack use the 34-element Sprag, or use the 8 element Spring-Roller.

Use a darn good TQ converter, matched for you application.

Use a Big- Car case. Use a rubber vacuum hose to extend the vent tube to be at the pan lower edge, tuck behind the Vac Mod.

Use a red-stripe adj Vac Modulator.

Use the original BW frictions smooth in the FWD pack, radial groove in the Dir pack. 5&5 or 5&6 is just fine. Use 3 original BW frictions in the INT stack.

For Reference the 68-72 Olds and Caddys have the best TH400 internals. 4L80s have some nice internals from FWD Dir Pack to Rear Planetary.

Use the Kick down switch, and stock Shifter.

[GoGoat]

There also is different length tailshafts.

[Schurkey]

My understanding--and I have not confirmed this--is that the lower the alphabetic descriptor on the tag, the more "bread-and-butter" the transmission calibration is.

So, for Pontiac, the alphabetic descriptor starts with "P". Then we have the variations: PA, PB, PC...and so on. A "PA" would be REALLY common, the performance/specialized versions--more torque capacity, higher shift points, anything else "specialized", would be higher-up the alphabet.

[West Coast GTO]

OK, I now understand some of the differences. I have a 67 TH-400 that had the long tail shaft( has been replaced) in my 71 455 GTO. The car has never shifted great and now slips/surges terribly. I have the opportunity to buy a 71 PX trans and I would think that would be a better match for my car.
Any thoughts?

Originally Posted by Half-Inch Stud

As for Pontiac TH400s, there are about 5-6 differences in VBs since 1965-66, and about 1-2 VBs since 1970. There will be differences in Separator plates, like under 10.

As for the rest of GM, more differences, but not many.

As for the internal parts, same deal where each part has a variant through the years, and some key parts have significant quality change:

Prefer the AH or A VBs, with the V ) V or V V Separator plates. No skift kit hole changes needed, just use only the 3 checkballs that lay along the centerline.
Flip the 2-3 accume in the VB.

Do the Direct Pack lip middle seal removal, 2nd Center support ring removal, and use an 0-80 screw-nut to block the tiny Reverse Bleed hole in the separator plate.

Inspect the Front Pump gearset condition, use a 121, or 893 pump assy, shim the baby blue Regulator Spring, or use an Orange Reg Spring.

Direct Pack use the 34-element Sprag, or use the 8 element Spring-Roller.

Use a darn good TQ converter, matched for you application.

Use a Big- Car case. Use a rubber vacuum hose to extend the vent tube to be at the pan lower edge, tuck behind the Vac Mod.

Use a red-stripe adj Vac Modulator.

Use the original BW frictions smooth in the FWD pack, radial groove in the Dir pack. 5&5 or 5&6 is just fine. Use 3 original BW frictions in the INT stack.

For Reference the 68-72 Olds and Caddys have the best TH400 internals. 4L80s have some nice internals from FWD Dir Pack to Rear Planetary.

Use the Kick down switch, and stock Shifter.

[Charlie Brengun]

Is there any gaurantee the trans has never been opened up, no one up or downgraded it with some shiftkit etc?

A used trans in my limited opinion is a complete gamble on what you are going to get.

[Half-Inch Stud]

Originally Posted by Schurkey

My understanding--and I have not confirmed this--is that the lower the alphabetic descriptor on the tag, the more "bread-and-butter" the transmission calibration is.

So, for Pontiac, the alphabetic descriptor starts with "P". Then we have the variations: PA, PB, PC...and so on. A "PA" would be REALLY common, the performance/specialized versions--more torque capacity, higher shift points, anything else "specialized", would be higher-up the alphabet.

Naw. PA, PB, PC are PMD Long-Tails, PC being the most stout.

PD-PZ would be shorttails. PD, PQ, PZ being less stout than the PC.

[Half-Inch Stud]

@west coast gto; what tag code is the longk tail? Time for filter replacement, and Vac mod tune.

PX internals not so special to me, except the valvebody and separator plate are fine. Uy the PX, and tear both down, use the best parts from both. Pro-hint; you are gonn use the Big Car case. Buy and Gasket and rubber kit, and follow my above post for best TH400 results.

[Dragncar]

Originally Posted by Half-Inch Stud

As for Pontiac TH400s, there are about 5-6 differences in VBs since 1965-66, and about 1-2 VBs since 1970. There will be differences in Separator plates, like under 10.

As for the rest of GM, more differences, but not many.

As for the internal parts, same deal where each part has a variant through the years, and some key parts have significant quality change:

Prefer the AH or A VBs, with the V ) V or V V Separator plates. No skift kit hole changes needed, just use only the 3 checkballs that lay along the centerline.
Flip the 2-3 accume in the VB.

Do the Direct Pack lip middle seal removal, 2nd Center support ring removal, and use an 0-80 screw-nut to block the tiny Reverse Bleed hole in the separator plate.

Inspect the Front Pump gearset condition, use a 121, or 893 pump assy, shim the baby blue Regulator Spring, or use an Orange Reg Spring.

Direct Pack use the 34-element Sprag, or use the 8 element Spring-Roller.

Use a darn good TQ converter, matched for you application.

Use a Big- Car case. Use a rubber vacuum hose to extend the vent tube to be at the pan lower edge, tuck behind the Vac Mod.

Use a red-stripe adj Vac Modulator.

Use the original BW frictions smooth in the FWD pack, radial groove in the Dir pack. 5&5 or 5&6 is just fine. Use 3 original BW frictions in the INT stack.

For Reference the 68-72 Olds and Caddys have the best TH400 internals. 4L80s have some nice internals from FWD Dir Pack to Rear Planetary.

Use the Kick down switch, and stock Shifter.

I run a T400 out of a 68 Olds station wagon and another out of a 70 Pontiac big car.
Has all the Trans Go kit and tricks along with a aluminum body J&W T brake.
Still has the long tail shaft in my GTO.
What makes the Olds trans better ?
I know where a `1980 Caddy is sitting, I can grab the T400 out of it.

[Gach]

Just a idea….One thing I would check is timing make sure its not advancing, I’ve seen over heating issues cause by timing keeps advancing up to 50 degrees. In other words, let’s say your total timing is 36 degrees at 2500 rpms. Check it with a timing light and make sure its not advancing up to 40-50 degrees as the rpms go past 2500 rpms. It just might explain temperature dropping as you slow it down then go up as you speed it up.

[Half-Inch Stud]

@ DragNcar the 68 Olds will have 3 high density torringtons, Front pump has higher PSI spring, a decent LO-band release spring, and HD case. Might even have 3/8" cooling lines.

Meanwhile the friction sets are 5&5&3 (a good standard).

@ Lou, yea huh

[West Coast GTO]

HIS,
The only code is the stamped info on drivers side, the (P) code plate is missing.
I have the trans go kit and a new adjustable engine vacuum valve.

Originally Posted by Half-Inch Stud

@west coast gto; what tag code is the longk tail? Time for filter replacement, and Vac mod tune.

PX internals not so special to me, except the valvebody and separator plate are fine. Uy the PX, and tear both down, use the best parts from both. Pro-hint; you are gonn use the Big Car case. Buy and Gasket and rubber kit, and follow my above post for best TH400 results.

[Cliff R]

The Transgo kit is excellent, assuming you bought the better kit with the direct drum return springs, "Jack Armstrong" retainer for case to retain the intermediate clutch pack, etc. I like those kits because they make it stupid simple to double feed direct without leaving a bunch of parts out of the build and tossing out half the check balls. I'd add here that there is nothing wrong with setting up a TH400 to double feed direct with "home-brew" methods, but one must realize that they made quite a few different seperator plates, and VB modulation differences as well. So unless you know which holes to drill in the separator plate and to what sizes, you can get varying results with the final product. Using a Transgo kit pretty much takes the quesswork out of the equation....FWIW.....

[Half-Inch Stud]

@ west coast gto; its an OA, OB, or OC. 5/16 cooling lines for OA, OB, 3/8 for OC.

[Dragncar]

Originally Posted by Cliff R

The Transgo kit is excellent, assuming you bought the better kit with the direct drum return springs, "Jack Armstrong" retainer for case to retain the intermediate clutch pack, etc. I like those kits because they make it stupid simple to double feed direct without leaving a bunch of parts out of the build and tossing out half the check balls. I'd add here that there is nothing wrong with setting up a TH400 to double feed direct with "home-brew" methods, but one must realize that they made quite a few different seperator plates, and VB modulation differences as well. So unless you know which holes to drill in the separator plate and to what sizes, you can get varying results with the final product. Using a Transgo kit pretty much takes the quesswork out of the equation....FWIW.....

I bought a kit for each transmission. Getting those springs in is no joke. I took a piece of 2 1/2" pipe and cut out enough of it so I could get that snap ring on.
I use a old scissor jack and compress it between the garage floor and the frame. Works great.
I have a complete Speedmaster input shaft, intermediate shaft, steel forward and direct drums and a billet center support to put in the T Brake trans at some point.
Trans Go makes good stuff.
I bought your book and use the Sessions book also.

[AG]

C clamps work well for that.

[Dragncar]

I have piles of C clamps. Just thought I would make one with stuff from work. Something to hang on the wall and use it now and then.
My Speedmaster stuff came without clutches and steels. I could have gotten them with clutches and steels but I figured since my hard parts are offshore made I might as well use USA made Borg Warner parts for the rest.
Took it all by a local tranny shop and the owner could not find anything wrong with them.

[West Coast GTO]

OK, I have set an appointment with the local mechanic specializing in TH400 trans work and he is going to install the Trans Go kit. I also have a torque converter thru Cliff on it's way to change out. So, should I just have the mechanic rebuild the trans since it has to come out( mostly) anyway for the converter?

[Dragncar]

You can pull the front pump and inspect the clutches and steels. If the clutches are in good shape, no burnt spots on the steels and the fluid does not smell like burnt toast you could just run it with the kit. Not hard to do. Pull the big snap ring on the forward clutch and take a look.
Otherwise it can cost a few bucks to replace all the bushings, seals, rings and clutches and steels.
I would get a screen type filter in it too. 32 element sprag while you are at it.