TH400 Information Thread

[NickTransmissions]

Hi All,

Since many of us own at least one GM Squarebody truck with a TH400, I figured I'd start another all-encompassing thread, this time on the Turbo Hydra Matic 400 aka THM400...I have basic facts, links and general information along with videos covering tear down and inspection, step by step rebuild procedures as well as high performance modes and go from there. Additionally, I have created a 'THM400 Fast Facts' reference sheet that I'll continuously update over time as I'll lose the ability to make edits to this first post after about 90 days or so. Much of the content below comes from the Fast Facts sheet.

Hoping this can be an easy 'one-stop shop' for exchanging information, guidance and advice as it relates to these units.

Special thanks to @Matt69olds for his input and contributions (see the various citations for his contributions throughout the Fast Facts sheet).

TH400 Family Production Run: 1964-1991
Variants:

• THM375 (1972-1976) - Lighter duty; 27 spline output shafts
• THM400 (1964-1994)
• THM475 (1971-1990)

Predecessor Transmissions: Powerglide, Hydramatic, Jetaway
Designation Change: 3L80; 3L80-HD (1991)

Matching Turbo Hydra Matic 375s, 400s and 475s with Associated Applications
All TH400 variants come from the factory with a metal tag riveted in place on the passenger side of the unit. This tag contains a two-digit application code along with other codification that can be used to determine the original year, make and model vehicle it came from, if that information is not already known to you.

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Click here for the THM400 Transmission ID Code Guide

Copy-paste this URL into your browser search bar if the above link isn’t working: https://docs.google.com/spreadsheets/d/1lA0a4yL1GV9bdd_5UbUVoNFVSsBNVwEZD2dDyrg3rhk/edit#gid=0

TH475 Teardown and Inspection

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TH475 Rebuild Series

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TH400 Valve Body Reassembly

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TH400 Full Playlist

https://www.youtube.com/playlist?list=PLMqgQ_reThtLJPRaV0iXcYkfDgYj2QBec

Fluid Capacities

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Fastener Torque Specifications

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Changes Through the Years: Pan - Case Belly
The below table is an excerpt from the 'Fast Facts' doc showing all changes that were made to the TH400s starting with the pan and filter designs and concluding with the case belly (see the Fast Facts doc for a full list of all changes - I'll update/correct/revise as I become aware of any new or incorrect information).

Vintage	Variant	Components	Interchange Information	Additional Context
1964-E1967	All	Oil Pan Filter	Early style filter not interchangeable with later style pan and vise versa	Early pans had three dimples and a ‘heel print’ contour on the bottom to accommodate the thicker early-style filter Early style filters contained a bypass valve
L1967+	All	Oil Pan Filter	Late style filter not interchangeable with early style pan and vise versa	Late pans have two dimples
1964	TH400	Valve Body Rooster Comb Selector shaft	- Stand-alone year - will not interchange with any other year and not ideal for hot rods	Single Range design, provisioned for P, R, N, D,1 - no manual 2nd range setting - No provision for retaining ‘nail’ in selector shaft - No pin in 2-3 accumulator bore
1965-1967	TH400	Torque Converter Pump TCC Stator solenoid	Interchangeable between all like-equipped vehicles within vintage	Switch pitch converters were variable stall converters that went from 2800 upon acceleration from low to high speed to appx 2000 for cruising, optimizing fuel economy. Installed in some Buick, Oldsmobile and Rolls Royce passenger cars
1964-1965	TH400	Valve Body	Provisioned for 12 valve body-case bolts - interchangeable with 10-bolt valve body/cases	First year of dual range design - provisioned for manual 2nd gear along with all other ranges (P,R,N,D,2,1)
1966-1990	TH400	Valve Body	Provisioned for 10 valve body-case bolts - will interchange w/1965 12-bolt valve bodies	No leaks will occur if you leave two bolt holes empty when bolting a 1965 valve body to a 66+ case
1964-1966	TH400	Valve Body	1-2 shift valve adjusting screen	Check height prior to removal
1964-1966	TH400	Valve Body Filter	Early-style filter used and will not interchange with later units (1967 and up)	No bolt hole location for filter as these did not take a bolt-on filter
1967-1990	All	Valve Body Filter	These valve bodies are backwards-compatible with 1965-1967 units w/out filter bolt	Filter design change to enable filter to be bolted to valve body; valve body updated w/filter bolt hole location
1971-1974	TH400	Valve Body TCS Pressure Switch	TCS = Temperature-controlled spark; these valve bodies were equipped with a transmission-control spark switch	Emissions-control related system
1964	All	Band Servo 2-3 Accumulator Piston	No functional relationship - not interchangeable with 65+ cases or valve bodies
1965-1970	All	Band Servo 2-3 Accumulator Piston	Interchangeable with 71+ as a service pack (servo and 2-3 acc piston)	Servo has half-moon shaped ridges 2-3 accumulator has flat top with recessed accumulator
1971+	All	Band Servo 2-3 Accumulator Piston	Interchangeable with 65-70 as a service pack (servo and 2-3 acc piston)	Servo has flat top 2-3 accumulator has three raised ridges
1978+	All	2-3 Accumulator Piston	Interchangeable with like-kind aluminum piston or with early design as a service pack w/the 2nd design servo and pin assembly	First year of plastic piston - these should be replaced with aluminum versions on overhaul as the plastic pistons often crack at the e-clip
1988+	All	Case Separator Plate Valve body Gaskets	Interchangeable provided that an early separator plate to case gasket is not used with a late case and 7th check ball OR where a late case is used and 7th check ball is left out when an early separator plate is used on the late case. The 7th check ball slows the flow of apply oil into the reverse circuit, softening the engagement from drive to reverse.	Per CK Performance regarding installing new vb/plate/gaskets to earlier case, “Any combination of early or revised transmission case, valve body, separator plate, or separator plate to valve body gaskets may be used as long as the revised separator plate to transmission case gasket is never installed on an early case, and the #7 checkball is not installed in the revised transmission case with the early separator plate in use. (Kokonis 6)

See 'Fast Facts' for the remainder of the table.

Specialty Tooling & Workarounds

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Parts Selection and Updating

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Performance Modifications
I very rarely use shift kits for most transmissions as much (if not more) can be accomplished without spending money on them, especially if you have the transmission apart on the bench.

Procedure	Location	Instructions	Addl Info
1-2 Accumulator valve mods	Valve Body	Single Valve: Leave out the spring for a firmer 1-2 shift; move the spring inboard for a very firm 1-2 shift (rec for high stall only) Dual Valve: Leave out the secondary valve spring (first one in the bore) for firmer 1-2 shift Later TH400 valve bodies contain one variation of the single valve configuration: Single valve, spring outboard (mild) Single valve, no spring (middle firm) Single valve, inboard (firmest)	Per, Chris Kokonis/CK Performance: Accumulator valve controls 1/2 shift feel.soft shift =a spring outboard of the valve, medium firm shift great for most applications=no spring on either side of the valve,super firm = a spring inboard of the valve(you can use a 400 int return spring in there if you like but usually too firm for my liking).Keep feed hole down to 110 or the accumulator valve mods don't result in changes you can feel or shift shock.then there is the 1/2 accumulator spring in the servo.no need to play here it doesn't do much when you do what was previously mentioned.These mods are for auto shift obviously. Post #236, TurboBuick.com (Kokonis) Per Matt69Olds on 1-2 acc spring setups:: Leave out the spring. Use a piece of copper tubing about an inch long, drop the tube into the bore, install the valve, the bore plug, then use a punch and tap the bore plug just enough to reinstall the roll pin. If you want to experiment with different springs, supposedly the return spring (Matt69Olds)
Full manual valve body (Courtesy of Matt69Olds)	Valve body	Grind the largest land off the 1-2 shift valve Plug the exhaust vent hole on top of the valve body over the 1-2 shift valve train (Stop here if you just want 'gear command' to hold manual low at any speed but want full auto pattern shifting otherwise) Braze shut the feed hole in the plate to the detent solenoid Loop governor oil and plug the governor tube holes or weld both weights in the "out" position and reinstall the governor Delete the vacuum modulator (which needs additional modifications to vent reverse boost oil) then grind the largest land off the modulator valve, or braze shut the “gov to mod” hole in the plate*.	*If this step is not completed, gov oil pressure will lower line pressure.
2-3 Accumulator delete	Valve Body	Drill and tap 2-3 accumulator feed hole in valve body ⅜ x16 then thread set screw w/loctite to block accumulator	Reinstall piston and e-clip.
Clutch feed orifice sizing	Separator plate	Drill your 1-2 and 2-3 shift feed orifices based on application, stall, final drive, 3rd accumulator treatment, preferred shift feel, etc..	General guidance/starting points - the specifics of your application and vehicle profile will ultimately dictate how you drill the plate: Moderate stall (2800-3400) - drill 1-2 .110 and 2-3 .125 to start, if 2-3 accumulator is kept active (do .115 if 2-3 accumulator circuit is blocked off) High Stall (4500+) - drill 1-2 .125 and 2-3 .140 to start
Additional Horseshoe spacer	Pump Cover	Add one more horseshoe spacer to the first one so that you can increase line pressure by appx 5% across the board	This will duplicate the procedure many shift kits will have you do with a washer included in the kit These can be found in at most transmission supply shops
Rollerize Forward hub (Courtesy of Matt69Olds)	Forward Drum	Machine the forward clutch hub for the 3-piece bearing that goes under the center support	There’s a roughly .030” thickness between the bearing assembly and forward hub’s thrust washer
Direct Clutch dual-feed procedure	Direct clutch, center support and case	Leave center seal off direct drum Leave 2nd from top sealing ring off center support stator Plug high-reverse feed port in case using ⅜” cup plug OR weld the high-reverse feed orifice in the separator plate (you can do both but it’s typically not necessary) Note - I don’t advise altering the center support itself as such alterations are permanent (some like to plug the high-rev port w/a check ball)	This mod should be done for anything high performance or heavy duty, high RPM or where HP >= 500. Procedure is the same for the 4L80E.
Direct Clutch	Direct Clutch Exhaust	Drill a .050-.065 bleed hole in the direct drum to prevent centrifugal apply at high rpm when driving through the direct clutch in first and second gear This should be done in conjunction with installing 16 high-rate return springs in place of the stock springs - they (and the same springs for the intermediate clutch) can be sourced from CK Performance	Measure from the outer diameter of the drum in .410-.420 inch, and drill a 1/16 inch hole. If you measure correctly, that will position the hole just inside of the l piston seal surface. The hole will provide an escape path for the trapped oil. (Matt69olds)
Intermediate sprag outer race	Direct Drum	Sand the inner race with 400-600 grit sandpaper or equivalent emery cloth to reintroduce cross-hatching.

I'll update this post with new information as I go until I'm no longer able to. I'll then bump the thread linking to changes made in the 'Fast Facts' sheet.

Post any suggestions, changes or corrections and I'll revise accordingly.
Otherwise, feel free to ask any TH400-related questions and I, @Matt69olds and others will respond!

 

[MarineOne]

Great compilation of info

 

[Catbox]

Awesome list of stuffs.

 

[Travlr]

Hi All,
Post any suggestions, changes or corrections and I'll revise accordingly.
Otherwise, feel free to ask any TH400-related questions and I, @Matt69olds and others will respond!

I hope this isn't hijacking the thread.

I was wondering if anyone has used a four speed Hydramatic trans and tossed the turbo 350/400 from their truck? The Hydramatic was used in heavy/light trucks as well as passenger cars for many years and I think it was a precursor to the 350/400 trannys, but I suspect was discontinued due to manufacturing costs.

 

[NickTransmissions]

I hope this isn't hijacking the thread.

I was wondering if anyone has used a four speed Hydramatic trans and tossed the turbo 350/400 from their truck? The Hydramatic was used in heavy/light trucks as well as passenger cars for many years and I think it was a precursor to the 350/400 trannys, but I suspect was discontinued due to manufacturing costs.

Interesting but you might start a separate thread on it to get some substantive replies as I'd like to keep this one focused on the TH400s.

 

[Matt69olds]

I just re-read this again, and thought of a couple other additions.

I prefer to install return springs in all 16 spring pockets in the direct drum. I do this to prevent centrifugal clutch apply. The direct drum spins almost twice engine rpm in 1st gear, the tiny amount of fluid left in the drum will “climb” towards the outer edge of the drum, because of the speed that trapped oil will cause the drum to partially apply. Probably not enough that you would ever notice it, but you will see hot spots on the steels plates on teardown. TransGo includes heavier return springs to help prevent this.

For really high rpm use, install all 16 springs, and drill a vent hole in the drum. Measure from the outer diameter of the drum in .410-.420 inch, and drill a 1/16 inch hole. If you measure correctly, that will position the hole just inside of the l piston seal surface. The hole will provide an escape path for the trapped oil.

The other item:intermediate clutch clearance. From the factory the clutch has a lot of clearance. The sloppy clearance tends to shock the lugs in the case that hold the snap ring in place. The shock from firmer shifts, added line pressure and the fact that these transmissions are at least 30 years old can result in the lugs blowing out of the case.

I prefer to tighten the clearance up to around .040-.050ish. The easiest way is to use a Chrysler 727, 518/618 front clutch snap ring. It’s the same outer diameter as the factory part and drops right in, but has the advantage of different thickness. The factory snap ring measures .080, the Chrysler snap rings are available in .010 increments starting at .068, and ending at .108. Supposedly the 4th clutch snap ring in a 2004R is the same snap ring, but I have never tried one to confirm.

BTW, the Chrysler snap ring is the same part included in the trans go shift kit.

Nick has put together the most complete tech and interchange article I can recall reading. You would have to buy several books and service manuals to equal it.

 

[NickTransmissions]

I just re-read this again, and thought of a couple other additions.

I prefer to install return springs in all 16 spring pockets in the direct drum. I do this to prevent centrifugal clutch apply. The direct drum spins almost twice engine rpm in 1st gear, the tiny amount of fluid left in the drum will “climb” towards the outer edge of the drum, because of the speed that trapped oil will cause the drum to partially apply. Probably not enough that you would ever notice it, but you will see hot spots on the steels plates on teardown. TransGo includes heavier return springs to help prevent this.

For really high rpm use, install all 16 springs, and drill a vent hole in the drum. Measure from the outer diameter of the drum in .410-.420 inch, and drill a 1/16 inch hole. If you measure correctly, that will position the hole just inside of the l piston seal surface. The hole will provide an escape path for the trapped oil.

The other item:intermediate clutch clearance. From the factory the clutch has a lot of clearance. The sloppy clearance tends to shock the lugs in the case that hold the snap ring in place. The shock from firmer shifts, added line pressure and the fact that these transmissions are at least 30 years old can result in the lugs blowing out of the case.

I prefer to tighten the clearance up to around .040-.050ish. The easiest way is to use a Chrysler 727, 518/618 front clutch snap ring. It’s the same outer diameter as the factory part and drops right in, but has the advantage of different thickness. The factory snap ring measures .080, the Chrysler snap rings are available in .010 increments starting at .068, and ending at .108. Supposedly the 4th clutch snap ring in a 2004R is the same snap ring, but I have never tried one to confirm.

BTW, the Chrysler snap ring is the same part included in the trans go shift kit.

Nick has put together the most complete tech and interchange article I can recall reading. You would have to buy several books and service manuals to equal it.

Thanks a bunch, Matt!

I updated the OP with some of the above (the rest will go onto the Fast Facts doc)...I also recommend Sonnax case savers for the intermediate clutch pack in HP/Race applications to further protect those int clutch lugs in TH400s and 4L80Es.

Went into the drawers and pulled out a 200-4R OD clutch snap ring and A518 Direct or forward clutch snap ring, compared them side by side and on top of each other...Theres a little bit of a difference but I imagine the slightly larger 200-4R snap ring would prob work, even if the ends had to be filed to fit if they ended up overlapping. The 200-4R snap ring is .093 which is a good, middle-of-the-road thickness...

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I also use high-rate return springs in the intermediate clutch if the application calls for super high RPMs and those can be had via CK Performance. Kolene coated or upgraded apply plates for intermediate and direct are also a good upgrade for the same situations.

 

[Sgt Gus]

I think I would drive to Vegas or Indiana, with our TH400, LOL.
Awesome of you to share y'alls knowledge!

 

[HotRodPC]

Thanks for the detailed addition Nick. I see in similar threads, you've added the same type threads for Th350 and 700r4 Also.

While on the subject of Th400, if anyone has or trips accross Ron Sessions book,

How to Work With and Modify the Turbo Hydra-Matic 400 Transmission, ​

like at a garage sale, book swap event, estate sale or whereever that you can pick up cheap, like $10-$20 or even less, pick that sucker up. It's a great book and is out of print. About the only place I know to find it now is on eBay and it's usually $100 or more used. I was lucky enough to score one a few years ago for like $10 that someone didn't know what they had. Even if you know Turbo 400's, I like it for the Identification Charts. There's great info in it about year model changes, which parts are best and which parts interchange, what year models have the more rare better parts like the dog bone sprag drums. Just get it if you happen to come across one cheap.

 

[NickTransmissions]

Thanks for the detailed addition Nick. I see in similar threads, you've added the same type threads for Th350 and 700r4 Also.

While on the subject of Th400, if anyone has or trips accross Ron Sessions book,

How to Work With and Modify the Turbo Hydra-Matic 400 Transmission, ​

like at a garage sale, book swap event, estate sale or whereever that you can pick up cheap, like $10-$20 or even less, pick that sucker up. It's a great book and is out of print. About the only place I know to find it now is on eBay and it's usually $100 or more used. I was lucky enough to score one a few years ago for like $10 that someone didn't know what they had. Even if you know Turbo 400's, I like it for the Identification Charts. There's great info in it about year model changes, which parts are best and which parts interchange, what year models have the more rare better parts like the dog bone sprag drums. Just get it if you happen to come across one cheap.

You're welcome, man -thanks for having me on the forum. And yes, Ron Sessions books on the TH350 and TH400 are the 'bibles' for those transmissions. I was lucky enough to spot a listing on eBay for both of those books several months back, seller only wanted $75.00 shipped so I jumped on it...A sizable fraction of the information in my TH400 fast facts sheet comes from Ron Sessions' TH400 book.

Used book stores are a a great place to go hunting for them as well.

 

[HotRodPC]

I came back to this thread to Stick it too. I seen you have some threads to add after Christmas. By all means, at your liesure. If you have the info, another great one either a thread all on it's own, or included with 4L80-E info, is the conversion or upgrade process from Th400 to 4L80-E. In my 85 K10/20 truck, it'll be going together with the Th400, but eventually if I get that far with that truck, I want to convert it to 4L80-E and IIRC, that can be done on a 4x4 model using a 2wd with an output shaft modification.

 

[NickTransmissions]

I came back to this thread to Stick it too. I seen you have some threads to add after Christmas. By all means, at your liesure. If you have the info, another great one either a thread all on it's own, or included with 4L80-E info, is the conversion or upgrade process from Th400 to 4L80-E. In my 85 K10/20 truck, it'll be going together with the Th400, but eventually if I get that far with that truck, I want to convert it to 4L80-E and IIRC, that can be done on a 4x4 model using a 2wd with an output shaft modification.

I believe some folks simply chop the 2WD shaft to the length they need and bolt on a 4WD extension housing that mates to whichever transfer case they're using and replace the rear case pin-hole seal on the rear of the transmission case with the 4x4 cup plug which can be done without tearing into the transmission. The cup plug is a GM part and readily available (ACDelco 9427692). If this is not done, fluid will bleed into the extension housing. That pin hole seal lubes the extension housing bushing in 2WD 4L80Es.

The vehicle speed sensor would remain in the Tcase though the 2WD unit will still have the rear speed sensor and corresponding reluctor 'gear' on the rear planet but it would be non-functioning for the most part. 1991-93 model year 4WD 4L80Es had three speed sensors total, two in the trans and one in the T-case for vehicle speed while 1994+ did away with the rear speed sensor in 4x4 4L80Es.

 

[squaredeal91]

I once cut the shaft on a 2wd TH350 and stuck a transfer case on it. Think I cut like 2" off.

 

[Catbox]

I would love a 4L80 version of this list as I am planning to one day put one into my truck behind the 454.
Awesome write up and information share.

That is a great way to earn customers.

 

[HotRodPC]

I believe some folks simply chop the 2WD shaft to the length they need and bolt on a 4WD extension housing that mates to whichever transfer case they're using and replace the rear case pin-hole seal on the rear of the transmission case with the 4x4 cup plug which can be done without tearing into the transmission. The cup plug is a GM part and readily available (ACDelco 9427692). If this is not done, fluid will bleed into the extension housing. That pin hole seal lubes the extension housing bushing in 2WD 4L80Es.

The vehicle speed sensor would remain in the Tcase though the 2WD unit will still have the rear speed sensor and corresponding reluctor 'gear' on the rear planet but it would be non-functioning for the most part. 1991-93 model year 4WD 4L80Es had three speed sensors total, two in the trans and one in the T-case for vehicle speed while 1994+ did away with the rear speed sensor in 4x4 4L80Es.

Yep, sounds exactly what I'm reffering to. Then of course add the stand alone controller or make your toggle switch shifter, or full manual VB.