T-400 Build Tips with Photos
This post is meant to help those who wish to rebuild their own transmission, however it is not meant to replace a quality rebuild manual.
I recommend using an ATSG rebuild manual, HP Books "How to Rebuild and Modify the TH400", and this post for those new to transmissions.
I will be covering some modifications that are not usually required but good to do.
After you have disassembled your trans and cleaned the mostly bare case, I recommend you change the shift shaft seal, kickdown 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 (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.
I use a universal cam bearing tool to replace many bushings in the various automatics. It will not do very small bushings. Pictured here driving the bushing out towards the front.
A TH400 and 4L80-E used in heavy towing will sometimes cause the rear bushing to "walk" to the rear, ruining the governor gear. There is a "fix" bushign for this that has a thrust surface on the front that prevents the bushing from walking.
I use a stock bushing, red loc-tited in place, and staked in place.
Last edited by jakeshoe; 09-18-2008 at 11:04 PM.
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 on top:
You set rear endplay using TH350 pump shims as pictured on the left here:
The total thickness of the bearing as compared to the thrust and selective is very close, it is usually about .150".
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.
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.
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:
It is best to have the center support already ready.
The center support houses the intermediate apply piston, 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.
Install the piston using a lip seal installed or feeler guage, be sure the piston depressions aling with the depressions in the center support, and it helps to use a tilting motion as you install it sometimes.
You can also use thin plastic (cut from a large plastic soda bottle) as a seal protector/installer or the proper Kent-Moore J-tools.
Once the intermediate apply piston is installed and the return springs, guide, retainer plate and snap ring, you must install the center support to direct drum selaing rings.
There are 3 choices here.
Solid teflon is on the left, then scarf cut teflon, then cast-iron.
Cast iron is what most kits will contain, and what was used stock in most TH400's.
Scarf cut teflon was used on later model Th400's.
Solid teflon is used on 4L80's and is interchangeable.
I prefer solid teflon, although it is the most time consuming type.
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.
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.
I use a piece of plastic (from a paint can cap) that is cut to size to install and size the rings. Leave it on until you are ready to install the direct drum.
Notice I left one sealing ring off, 2nd from top groove. This groove seperates 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.
Also of note,
Pictured is a late style reaction carrier 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 type for high-HP transbrake builds although not mandatory.
My "compressor/sizer" for the sealing rings.
Next, you will finish assembling the rear portion for installation into the case.
Install the center support into the reaction carrier.
You will install the rear thrust bearing into the case or the stock selective in the case with the stock thrust on the rear planet (using assembly gel, petroleum jelly, or grease 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 trans have this).
I install the part of the rear servo to help hold the band in place. Not always necessary but it helps.
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 position as pictured, these can be used to line it up properly.
Once the assembly is seated in the case, be sure the feed holes in the valve body area line up properly, the rear band appy 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 VB area, force the center support in a counteclockwise direction and tighten the center support bolt.
This picture shows a common roller clutch style drum and a roller clutch on the left, most TH400's will use this style drum, early units, pre-71 will have the smooth inner race style drum on the right. 4L80-E'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.
This is a pic of an upgrade for high rpm applications, it is a spirolock type retainer instead of a regular snap ring. The regular snap rings will release due to centrifugal forces at high rpms.
This part is a stock 4L80 part available at a GM stealership.
This is the intermediate clutch stack 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 luges here in the case are one of the weak points of a TH400, high line pressure, harsh shift calibration, and fatique 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 200-4R. Some performance vendors sell a brace that helps prevent case lug breakage.
One of the keys is to keep 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.
Next goes the forward drum:
Case is prepared for pump installation.
More to follow as I finish this build.
I will cover intermeidate cluitch installation, direct drum assembly, forward drum assembly, pump inspection and assembly, air checks, dual feeding modifications, 34 element intermediate sprag vs. stock 16 element and later roller clutch as well as various assembly tips to improve the TH400.
Making your own valve body kit.
The TH400 doesn't require anything super fancy to get a firm shifting unit.
You need to drill the feed holes shown in this picture. I recommend drilling them both to .125". The 3rd feed hole can go bigger, but I would recommend .140" as the high end.
You also block the 2-3 accumulator feed hole in the valve body. This requires a 5/16" set screw and proper tap. You will remove the 2-3 accumulator piston, discard the spring.
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.
More info for this thread to answer a good question.
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.
Simply grind two flats on the shift valve and deburr them.
You can plug the hole with a brass dowel or a checkball. To use a checkball you will have to first drill the hole larger and then drive the checkball in place.
Another mod I covered but did not photo 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.
I run 5 checkballs usually.
Locations are as follows.
Red X marks the spot
There is an Army Technical Manual posted here with specific step by step instructions for basic rebuild.
Halfway down the page, look for 3L80 links.
what pump mods do you do,i know some have had the thurst eat up in the eng from high line pressure .i have been told that cooler line pressrue is what to look at,under 50 psi is good thanks for the good info.
I restrict the converter feed in the pump. Tap the hole and install a plug drilled for a smaller opening. I drill it .100"
This thread is in the Crankshaft Coalition wiki .
clik here for T-400 tips
You can email jakeshoe or send a PM via the Hotrodders board.
At the Bonneville Salt Flats, first gear is known as 130 mph.
Last edited by Crosley; 08-31-2011 at 10:48 PM.
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