TH400 rebuild tech

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Contents

[edit] TH400 transmission rebuild tech

NOTE:

  • This information and photos are meant to help those who wish to rebuild their own transmission, however this is not intended to be a replacement for an ATSG or factory tech manual. First time builders will likely need a tech manual. Recommended is an ATSG rebuild manual, and Turbo Hydra-Matic 350 Handbook by Ron Sessions, and this post for those new to transmissions.

The original article by jakeshoe can be seen in the Hotrodders forum thread TH400 Build Tips with Photos.

Besides the basic rebuild, also shown here are some modifications that are not usually required, but are still good to do.

[edit] Shift shaft seal replace

After you have disassembled your trans and cleaned the mostly bare case, I recommend you change the shift shaft seal, kick down connector O-ring, and rear case bushing.

Pictured here is a tool that allows removing the shift shaft seal without removal of the parking linkage. It is relatively inexpensive and can be used without pan removal with the transmission in the car.

Be sure to lube the seal before re-installing, and take care not to cut it on the edges of the shifter shaft.

If you do not have the tool or wish to buy it, now is DEFINITELY the time to change the seal, it requires removal of the linkage. You want to do this before you have the trans internals assembled. If you drop the "nail" in the case, or anything else, it is very frustrating later on to disassemble.

Shift seal tool.jpg

Warning The following was added by user 184.71.102.174:

Note: I'm a builder with 35 yrs experience and we have a full machine shop with bore gauges. We do not replace any bushings unless absolutely necessary in any transmission. All the aftermarket bushings I have ever used except ones that are designed to be machined after installation, ALWAYS result in GREATER clearance that the OEM bushing your replacing. If you shrink them in dry ice and tap them in you will have better results but they still maybe larger than OEM. The only bushing we change consistently is the front pump. We prefer to find another part with a good bushing.

[edit] Main case bushing replacement

A universal cam bearing tool can be used to replace many bushings in the various automatics. It will not do very small bushings. Pictured here driving the bushing out towards the front.

Rearbushingdriver.jpg

A TH400 and 4L80E used in heavy towing will sometimes cause the rear bushing to "walk" to the rear, ruining the governor gear. There is a "fix" bushing for this that has a thrust surface on the front that prevents the bushing from walking.

Use a stock bushing with red loctite and staked in place.

If you plan on using a rear torrington-type bearing you want to place the bushing ever so slightly protruding to the front, so that it will locate the bearing.

I use a TH350 pump - direct drum bearing from a later style TH350. These can be ordered new (preferred). This torrington bearing replaces the stock thrust washer and rear selective tang washer in the TH400. Pictured here, bearing is at the top:

Rear-thrustcompare.jpg


You set rear end play using TH350 pump shims as pictured on the left here:

Rear-thrustshim.jpg


The total thickness of the bearing as compared to the thrust and selective is very close, it is usually about 0.150" (#25 bit, 0.1495").

Rear-bearing-measure.jpg

It is best to adjust rear endplay on the tight side. On a TH400 (unless it has straight cut planetary gears) the planets will try to thrust load against the case bearing and the center support.

[edit] Rear planetary setup

I disassemble the rear output shaft from the output ring gear and clean and inspect the planets. Refer to manual for checking procedures and specs. It is shown here re-assembled with the reaction carrier and sun gear ready to be re-assembled.

Rear planets.jpg


Slide the reaction carrier onto the rear planet assembly, drop in the sun gear, flat side toward the front of trans. Then slide the reaction tube in, shown to the rear:

Reaction tube.jpg


[edit] Center support assembly

It is best to have the center support ready ahead of time. The center support houses the intermediate apply piston, and supports the direct drum. It has sealing rings that seal it to the direct drum, allowing it to transfer fluid to the drum to apply 3rd gear, and reverse. Disassemble, clean, and re-assemble using new lip seals. Pay attention to the orientation. Use the old lip seals to be sure you select the properly sized new seals.

Center support rings.jpg


Install the piston using a lip seal installer or feeler gauge, be sure the piston depressions align with the depressions in the center support, and it sometimes helps to use a tilting motion as you install it. You can also use thin plastic (cut from a large plastic soda bottle) as a seal protector/installer, or use the proper Kent-Moore J-tools.

Once the intermediate apply piston and the return springs, guide, retainer plate and snap ring is installed, you must install the center support to direct drum sealing rings.

  • There are three choices here, shown below:
  1. Solid teflon is on the left
  2. Scarf cut teflon, center
  3. Cast iron, right

Seal rings t-400.jpg

  • Cast iron is what most kits will contain, and what was used stock in most TH400 transmissions. Cast iron is the easiest and will work fine for most applications but if using any type of valve body that will not require a modulator teflon is best as it will prevent wear on the direct drum.
  • Scarf cut teflon was used on later models of the TH400.
  • Solid teflon is used on the 4L80 series and is interchangeable. I prefer solid teflon, although it is the most time consuming type. If using teflon, be sure there are no burrs on the center support ring grooves. Installing the teflon rings requires a protector to prevent cutting or gouging them, and a sizing device.

It helps to heat the rings in hot water before installing so that they will stretch easier. A piece of plastic (from a paint can cap) that is cut to size can be used to install and size the rings. Leave it on until you are ready to install the direct drum.

Warning Note: One sealing ring has been left off, 2nd from top groove. This groove separates the direct (3rd gear) and reverse feeds from each other. When dual feeding the directs (as done in this build), this sealing ring is not necessary. Dual feeding the directs on a TH400 will be covered in more depth in another section.

Center support rings.jpg


Custom modification of the center support. The sealing ring is left off, the ring land has been machined off. This allows the oil to flow easier, quicker to the direct drum. Original unmodified center support is on the right in the photo.

T-400 custom center support.jpg


Photo below shows the thrust washer on the back side of the center support installed. It is held in place with some assembly grease.

T-400 center support washr.jpg


Pictured is a late style reaction carrier (left) and an early style. I believe the early style was only used on 1964-1966 models. Notice the actual sprag-type element used on the early style on the right. I prefer these types for high HP transbrake builds although not mandatory.

Planets.jpg


My "compressor/sizer" for the sealing rings. Radiator hose with band-type clamp. You also use a soda pop aluminum can section. Be careful- the thin metal can cut the teflon rings and fingers.

Center-support-compress.jpg


Next, you will finish assembling the rear gear train portion for installation into the case.

Install the center support into the reaction carrier. You will need to rotate the center support into the roller clutch assembly. Center support will then slide into the assembly.

Rearassembly.jpg


You will install the rear thrust bearing into the case or the stock selective spacer in the case with the stock thrust bearing on the rear planet (using assembly gel or petroleum jelly to hold in place). You will also install the low/reverse (rear) band. If required you install the fretting snap ring into the case. ATSG covers when this is necessary, most later model ('71-up?) transmissions have this.

Casefretring.jpg


I install the part of the rear servo to help hold the band in place. Not always necessary but it helps.

Barecase.jpg


Now you will need to lift the rear assembly and install. This rear assembly is relatively heavy. It requires you to lift by the center shaft, you cannot lift it by the center support as it will slide out of the reaction carrier.

I use a paper towel to pad it slightly, and grip the shaft between my middle and index finger. I lift it using one hand under the output carrier until I'm ready to set it into the case. Then all the weight is on your fingers for a few seconds. If you have a helper they may be able to hold the output shaft once it passes through the rear case hole and hole in the bench.

The center support must line up with the holes in the case, you can see two angle cut tangs on the center support in about the 8 and 10 o'clock positions as pictured, these can be used to line it up properly.

Rearhalf install.jpg


Once the assembly is seated in the case, be sure the feed holes in the valve body area line up properly, and that the rear band apply is in the correct location.

Install the center support bolt finger tight, then install the beveled case ring with the gap at the 9 o'clock position.

Then using a screwdriver through one of the feed holes in the valve body area, force the center support in a counterclockwise direction and tighten the center support bolt.

The center support snap ring has a taper on one side. The tapered side is installed up as you look into the case.

Next is the intermediate clutch pack to install into the case with the proper snap ring.

T-400 intermed clutch.jpg


[edit] Sprag design comparison

The next picture shows a common roller clutch style drum and a roller clutch on the left, most TH400 use this style drum. Early units, pre-'71, will have the smooth inner race style drum on the right. 4L80E's also use the "early" style drum and an actual sprag. The stock 16 element sprag is in the center, with the 34 element on the right.

The 4L80E direct drum uses a sprag race that is 0.045 to 0.050 wider than the TH400 sprag race. If you acquire a 4L80E direct drum be sure to have the correct race, too.

TH400 roller clutch and early design sprag unit

[edit] Spiro loc retainer

This is a picture of an upgrade for high RPM applications, it is a spiro lock type retainer instead of a regular snap ring. The regular snap rings will release due to centrifugal forces at high RPM. This fits on the direct drum plate that covers the sprag.

Spirolock.jpg

This part is a stock 4L80E part available at a GM dealership.

A stock snap ring can also be tack welded to the retainer. The TIG welding method with silicon bronze is best. Most DIY folks at home do not have the ability to use this method.

[edit] Direct drum modification

In the direct drum, leave out the center lip seal. This is the "dual feed" modification you read about.

TH400 direct drum, center seal left out; check ball circled


Notice the circle around the check ball in the aluminum piston. Each drum and piston assembly must have one of these check balls. The check ball may be in the piston, it may be in the drum as circled in the drum photo.

TH400 piston check ball location


You must install a full set of returns springs in the direct drum. Ignore the experts or online information to leave springs out of the direct drum. If you have a set of heavy duty return springs from a kit or supplier, install half HD springs with half OEM springs. This helps to avoid residual oil behind the piston from forcing the piston out at high RPM when in 1st or 2nd gears.

TH400 piston with springs


[edit] Clutch pack clearances

  • Direct and forward clutch drums:

0.050" to 0.080" in each drum works well with 5 frictions.

Typically the thicker steel plates are used in the direct clutch drum. These can be mixed with thinner steels used in forward drum when setting clearance. Try to maintain thicker steels in the direct clutch drum. Thicker steels take the heat of shifting better.

  • Intermediate clutch pack:

0.040" to 0.060" clearance in the intermediate clutch with 3 frictions. Always use a thicker snap ring 0.095"-0.100" to retain the pressure plate.

[edit] Full manual shift modifications

FULL manual shifts means just that. You must move the shifter for every shift up or down. NO Automatic shifts.

There are two steel governor fluid tubes out the rear of the main valve body. Remove these tubes. Plug the holes in valve body. 3/8-16 tap and set screws work. Use a liquid thread locker like Loctite.

Gov tube holes.jpg


A new 0.110" (#35 or 7/64" bit) hole needs to be drilled in the proper place to loop the governor fluid. Check photos here for location.

T-400 full manual loop03.jpg


You must also modify the 1-2 shift valve as listed below for FULL manual shift conversion:

The TH400 has a over-ride shift that will shift out of low gear even if you have manually selected low. It was designed to prevent engine damage. You can eliminate this feature by performing the following modifications to the valve body.

You must remove the 1-2 shift valve and grind two flats on the land designated by the arrow. You must plug the exhaust hole in the valve body also designated.

1-2 shift valve.jpg

Simply grind two flats on the shift valve and deburr them. You can plug the hole with a brass dowel or a check ball. To use a check ball you will have to first drill the hole larger and then drive the check ball in place.


Drill the marked holes to .125" (1/8") for street use. At the 1-2 shift hole location (designated #2), drill the small hole out.

For a heavy duty application: 3rd gear shift use a .150" drill (#25 bit, .149) and leave the 1-2 check ball out. Caution: A 34 element sprag needs to be used with the 1-2 check ball removed.

Near the bottom of the separator plate in photo, the hole in the round black circle (designated 'Dual Feed') should be plugged when the dual feed modifications are performed with the center lip seal removed from the direct clutch drum.

T-400 feed holes03.jpg


Another mod is blocking the 2-3 accumulator. I do this with a set screw. You tap the hole and install the set screw available from any hardware store. You will remove the 2-3 accumulator piston, discard the spring. This firms up the 2-3 shift.

Re-Install the piston in the hole. Do not use a plastic piston, they break.

Tap the hole that feed the backside, or spring side, of the accumulator piston. Install the set screw. Re-install the accumulator piston.

What this does is eliminates accumulator function on the 2-3 shift. This results in a very positive 2-3 shift. You could leave 2-3 accumulator function in place and go slightly bigger on the feed hole.

100 2218-1.jpg

[edit] Boost valve modification

(Full manual shift only)

Modification of the boost valve to have static line pressure and vacuum modulator replaced with aluminum plug. This modification of the boost valve avoids the run away high line pressure in reverse.

Grind a flat spot on the large spool valve land with arrow pointed at it. It is not fussy, 1/4" to 3/8" wide is fine.

400boost01.jpg


Hole location in case to plug. This is shown with the pump removed from the case. This passage must be plugged if boost valve land is ground with flat spot.

400boost02.jpg


Method #2 to plug passage in case: You can remove the valve body and plug the hole shown in photo if pump removal is not going to happen. Perform this method or the method posted above. Both are not needed.

You can slip a 1/4" check ball into the square hole and push it into the round passage at the base of this square hole.

Block rev hole.jpg


After the boost valve modification, you can install an aluminum plug that blocks the modulator valve in place. The original bolt and bracket from the vacuum modulator holds the aluminum plug in place.

T-400 modulator plug.jpg

[edit] Valve body mods, automatic shifts

The TH400 has a over-ride shift that will shift out of low gear even if you have manually selected low. It was designed to prevent engine damage. You can eliminate this feature by performing the following modifications to the valve body.

You must remove the 1-2 shift valve and grind two flats on the land designated by the arrow. You must plug the exhaust hole in the valve body also designated.

1-2 shift valve.jpg

Simply grind two flats on the shift valve and deburr them. You can plug the hole with a brass dowel or a check ball. To use a check ball you will have to first drill the hole larger and then drive the check ball in place.


Drill the marked holes in the separator plate to 0.125" (1/8") for street use. At the 1-2 shift hole location (circled #2), drill out the small hole.

For a heavy duty application: 3rd gear shift use 0.150" (#25 bit, 0.1495") drill and leave the 1-2 check ball out. Caution: A 34 element sprag needs to used if the 1-2 check ball is removed.

Near the bottom of the separator plate in photo, the hole in the round black circle (designated 'Dual Feed') should be plugged when the dual feed modifications are performed with the center lip seal removed from the direct clutch drum.

T-400 feed holes03.jpg


Another modification is blocking the 2-3 accumulator. I do this with a set screw. You tap the hole and install the set screw available from any hardware store. This firms up the 2-3 shift.

Re-install the piston in the bore. DO NOT use a plastic piston, they break.

100 2218-1.jpg


Next pull out the valve shown below. IF the spring is on the outer end as shown, remove the spring. This will firm up 1-2 light throttle shifts. For a firmer 1-2 shift, install a small spring on the inside of this valve. The OEM outer spring is too large to fit into the bore first.

T-400 acc valve.jpg


[edit] Drum installation

Then the intermediate clutch stack is assembled into the case on top of the center support. There is a snap ring that goes in the case above the pressure plate pictured. The splined case lugs here in the case are one of the weak points of a TH400. High line pressure, harsh shift calibration, and fatigue cause the lugs to break here. TransGo kits contain a heavy snap ring to help this, I use a center support snap ring from a TH2004R. Some performance vendors sell a brace that helps prevent case lug breakage. One of the keys is to keep the TH400 line pressure under 250 psi if possible.

This is the direct drum sitting in the case. The sprag race engages the intermediate clutches. If using a valve body with engine braking, be sure you install the intermediate band.

Direct drum install.jpg


Next goes the forward drum:

Fwd-druminstall.jpg


Case is now prepared for pump installation.


[edit] Check ball locations

Install 5 check balls in the locations marked with a red X.

T-400checkballs.jpg


[edit] Governor

Photo of the TH400 governor and cover. Note the direction the plastic drive gear teeth. The plastic drive gear can wear out or break. This result in no upshifts since the governor is not rotating.

TH400 governor with cover

[edit] Vacuum modulator

The vacuum modulator senses vacuum in the intake manifold as an indicator of engine load. As the throttle is opened, vacuum decreases. The modulator on the transmission will raise the main line pressure in the transmission

Two types of modulators shown. Low vacuum design on left, ID by the black and red strips. It also has a larger diameter

[edit] Kickdown wiring

TH400 kick down wiring.jpg

In the diagram above (1969 Camaro), the power is taken at the wiper motor by using a piggyback terminal connector.

The kickdown was originally switched either at the accelerator pedal or at the carb throttle linkage, depending on make. A microswitch can be used to trigger the kickdown, or a factory switch can be used. B&M also sells a kit that includes a heavy duty switch and wiring (shown below).

Bm-20297.jpg

[edit] Line pressure

Range Minimum Maximum
Park 55-70 150-175
Reverse 95-150 235-260
Neutral 55-70 150-175
Drive 70-90 135-165
Second 135-165 135-165
Low 135-165 135-165

Note: Line pressure fitting is located on driver side of transmission close to shift lever.

[edit] Transfer case info

There are three different output shaft lengths for NP transfer case applications. Adapter, transfer case, and output shaft length must all match.

Output shaft lengths (from the back of the case to the end of the shaft)

Output shaft lengths (from the back of the case to the end of the shaft)...

  • TH400 to NP203 = 1-3/8" (1.1" adapter)
  • TH400 to late NP205/NP208 = 2-1/2"
  • TH400 to early NP205 = 4-1/2" (same as short 2WD output)
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