Editing TH400 rebuild tech

{{youcanedit}} =Rebuilding the T-400= ===Clearance in clutch packs:=== *Direct and forward clutch drums: .050 to .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: .040 to .060 clearance in the intermediate clutch with 3 frictions. Always use a thicker snap ring .095 - .100 to retain the pressure plate ===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 v-body. 3/8 x 16 tap and set screws work. Use liquid thread locking chemical [[image:400v-body6.jpg]] A new hole in the proper place needs to be drilled (.110) to loop the governor fluid. Check photos here for location [[file:T-400_full_manual_loop03.jpg]] ===Boost valve modification=== 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 inch wide is fine [[file: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. [[file:400boost02.jpg]] Method 2 to plug passage in case: You can remove the valve body and plug this 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 inch check ball into the square hole and push it into the round passage at the base of this square hole. [[file:400boost03.jpg]] After the boot valve modification, you can install an aluminum plug that blocks the modulator valve in place. [[file:T-400_modulator_plug.jpg‎ ]] =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. [[file: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 at .125 for street use. For a heavy duty application : 3rd gear shift use .150 drill and leave the 1-2 check ball out. Caution a 34 element sprag needs to used with the 1-2 check ball removed. [[file:T-400feed-holes.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. This firms up the 2-3 shift. [[file:100_2218-1.jpg]] =Jakeshoe rebuild tech:= 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. ===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 (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. [[file:Shift_seal_tool.jpg]] ===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. [[file:Rearbushingdriver.jpg]] 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" bushing for this that has a thrust surface on the front that prevents the bushing from walking. Use a stock bushing, red loctite in place, 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 on top: [[File:Rear-thrustcompare.jpg]] You set rear end play using TH350 pump shims as pictured on the left here: [[file:Rear-thrustshim.jpg]] The total thickness of the bearing as compared to the thrust and selective is very close, it is usually about .150". [[file: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. ===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. [[file: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: [[file:Reaction_tube.jpg]] ===Center support assembly=== 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. [[File:Center_support_rings.jpg]] Install the piston using a lip seal installed or feeler guage, be sure the piston depressions align 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 sealing rings. There are 3 choices here. Solid teflon is on the left, then scarf cut teflon, then cast-iron. [[file:Sealing_rings.jpg]] 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. [[File:Center_support_rings.jpg]] 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. [[file: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 [[file: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 [[file:Rearassembly.jpg]] 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). [[file:Casefretring.jpg]] I install the part of the rear servo to help hold the band in place. Not always necessary but it helps. [[file: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 position as pictured, these can be used to line it up properly. [[file:Rearhalf_install.jpg]] Once the assembly is seated in the case, be sure the feed holes in the valve body area line up properly, 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 VB area, force the center support in a counterclockwise direction and tighten the center support bolt. ===Sprag design comparison=== This next 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. [[file:Sprag-drum-compare.jpg]] ===Spiro loc retainer=== This is a pic 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 rpms. This fits on the direct drum plate that covers the sprag [[file: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 access to this method ===Direct drum modification=== In the direct drum, leave out the center lip seal. This is the "dual feed" modification you read about. [[file:Th400centerseal.jpg]] 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 [[file:T-400_direct_piston2.jpg]] 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 [[file:T-400_direct_piston.jpg]] ===Drum installation=== 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. photo of intermediate clutch assembled in case needed Frictions , snap ring and pressure plate [[file:T-400_intermed_clutch.jpg]] 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. [[file:Direct_drum_install.jpg]] Next goes the forward drum: [[file:Fwd-druminstall.jpg]] Case is prepared for pump installation. 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. ===Check ball locations=== Install 5 check balls in the locations marked in red X. [[file:T-400checkballs.jpg]] [[Category:Transmission]] [[Category:good articles]]

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