Completely and correctly rebuilding an automatic transmission is not beyond the capabilities of the average automotive enthusiast. But there is something about it that keeps away even the most skilled automotive technicians. I always hear the same thing: “I don’t have all the special tools to rebuild them.” So the tranny is usually carried off to a specialty shop instead.
This Tech Tip is From the Full Book, HOW TO REBUILD & MODIFY GM TURBO 400 TRANSMISSIONS. For a comprehensive guide on this entire subject you can visit this link:
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It is true that a few tools, which may be called “special,” are required for certain operations I cover in this book. But the good news is that the majority of the tools required are simply ordinary hand tools. Even better news is that, with a little extra effort, you can improvise or fabricate a tool for most any particular operation.
Safety equipment is first and foremost on my list. Automatic transmissions use pressurized fluid. During a rebuild, you come into contact with transmission fluid and cleaning solvents. You also use compressed air for drying parts, and for air-pressure-testing clutch packs, servos, etc.
Eye protection is a must anytime compressed air is used. When cleaning parts, an injury can result easily and very painfully if high-detergent transmission fluid or a piece of debris is blown into your eyes.
Nitrile gloves are good for keeping solvents and transmission fluid off your hands. They also provide some protection from getting cut on the jagged edges inside the cases. The quality of the gloves currently available varies considerably. The better-quality gloves are thicker and more resistant to solvents such as brake cleaner, which works well to remove oily residue from parts before assembly.
Heavy-duty chemical-resistant gloves also provide protection. Wear them when cleaning the transmission case with heavy-duty degreasers or other harsh chemicals.
Hearing protection is also essential. During the rebuild, you may use compressed air to clean parts and dry them. When high-pressure air is blown into the case passages, for example, it can create a highpitched sound that is injurious to your hearing.
Sockets and Wrenches
General Motors used both metric and Society of Automotive Engineers (SAE) fasteners through the years of production. The later units used metric fasteners, but some units have SAE and metric on the same unit. A complete socket set, with 1/4-inchdrive, 3/8-inch-drive, and 1/2-inchdrive sockets are needed.
Make sure you have extensions for each drive. An air ratchet or an impact gun speeds up the rebuilding process considerably, but never use them to tighten fasteners because most of them are threaded into the aluminum case. The aluminum is soft enough that the threads may strip out if you use excessive torque. I prefer a spin-handle wrench for quickly installing bolts and tightening them down. They do not require compressed air and are much less likely to overtighten any fasteners.
A torque wrench is a valuable tool because each fastener size has a torque value based on its diameter and hardness. Still, using one does not make up for common sense, and the long handles on a torque wrench make it easier to overtighten a bolt and strip the threads out of the case. This is a greater concern with the smaller fasteners used to hold the valve body in place.
In addition to a complete socket set, you also need some hand wrenches. I especially like the new styles that have the ratcheting feature on the boxed end. They shorten the time typically required to tighten or remove a fastener that you cannot access with a socket.
Flare-nut or tubing wrenches are made specifically for accessing the nuts that hold steel lines in place. They have additional material and increased integrity to loosen stubborn flare nuts that are used to hold cooling lines to the case. They allow the wrench to slip over the tube and still get a good hold of the flare nut to remove it.
Snap Ring Pliers
You need a variety of snap ring pliers. Some of them must be modified slightly for a specific purpose.
You can also buy pliers with removable tips, so one pair can cover a broad range of applications. Some snap ring pliers are also designed to be convertible. They can be modified in seconds to remove either the inner or outer snap rings.
A variety of screwdrivers come in handy during a rebuild. Large flat-tip screwdrivers can be used to remove the snap rings in transmission cases, and also to put them back in position. Phillips screwdrivers also double as alignment tools; for example, when putting the pump back on the case. In a couple of minutes with a torch, you can take a couple of small flat-tip screwdrivers and turn them into “friction and steel” removal tools. By bending the ends over, you can use them to reach inside clutch drums or into the transmission case to pull steels and friction plates out. An awl also comes in handy for transmission rebuilding. You can use it to help line up bolt holes with gaskets and separator plates when installing the valve body, and to remove the snap ring that retains the 1-2 accumulator in the case.
Transmission rebuilding involves precision and attention to detail. Quite a few items within the assembly need to be measured for specification. A dial indicator is a valuable precision tool for transmission builds. It can be used to check inputor output-shaft endplay before and after the unit is assembled. Since you may have selective shims available to set the shaft endplay, it’s preferable to be able to measure all components accurately. It can also be used to measure the inside and outside diameter of bushings and other components. The other end also has a depth gauge. This handy part of the tool indicates how far down inside a drum a particular bushing was installed prior to driving it out for replacement.
A standard zero to 1-inch micrometer works equally as well for measuring frictions (friction plates) and steel plates, as well as the total thickness of a clutch pack.
Turbo-Hydramatic 350s require a piston spring compressor inside the case. Many years ago I made a spring compressor, in about five minutes, from two pieces of flat steel bar. It has served me well for decades and the total investment was only a few dollars.
For the Turbo-Hydramatic 400 (TH400), you can fabricate similar tools from steel flat plate and longer pieces of threaded rod. You can use these to compress spring cages on other components, provided they have a hole through the center of them. For drums that do not, hold the component stationary and use some sort of spring compressor to push down on the spring cage to the snap ring.
You need a larger bench-mounted spring compressor for building clutch drums. A shop press works equally as well, although they tend to be much more difficult to set up and a lot slower to use. You can fabricate a spring compressor from a few pieces of angle and square tubing. They are also available through most commercial tool supply stores. The unit I use was built from a few pieces of metal left over from other projects. It mounts easily in a large shop vise when I use it for clutch drums with a protruding shaft. You can drill a hole in your workbench to accommodate the protruding shafts of these drums if you need to, which allows the tool to be used on a flat work surface if a large vise is not available.
Case Holding Fixtures
A transmission holding fixture, if available, makes a build go much easier. One is certainly not required, but being able to turn the transmission into any position and lock it in place helps with the disassembly and reassembly processes. Several different types are available. Conveniently, General Motors cast bosses into the case to accommodate a special holding fixture. Two round pins engage the case, one on each side, and then a long threaded bolt is tightened against the top of the case. Since no oil pan bolt holes are used, the entire transmission can be taken apart and reassembled while in the holding fixture.
Most universal holding fixtures use a couple of bolts into the pan rails. They still provide stability for the unit, and the ability to lock it into any position, but the tool must be removed to install the oil pan and complete the rebuild.
If you are using a holding fixture of any type, it needs to be securely attached to a heavy-duty table or workbench. You may need to counterbalance the workbench to offset the weight of the transmission. You can construct a simple heavy-duty workbench from a couple of 4 x 8 sheets of plywood, 4 x 4s, several 2 x 6s, and some good wood screws. The benches I use in my shop have storage space underneath them. The weight of the parts on the lower shelf provides enough stability so the weight of the transmission doesn’t cause the table to fall over. The 8-foot work surface provides plenty of room to lay out all of the parts removed from the transmission.
Pans for Small Parts
Large commercial-grade cookie sheets make a great place to put the internals of the transmission as they are removed from the case. The cookie sheets hold the oil draining from the parts, and keep any small parts (such as check balls) from rolling onto the floor. Frictions and bands need to be soaked in clean automatic transmission fluid (ATF) for at least 15 minutes prior to assembly. You can use a cleaned 1.5- gallon ice cream pail, with about a quart of clean ATF, to soak frictions and bands. It has a lid to keep out dirt and debris. It also works well for lubricating Torrington bearings, and dipping apply pistons prior to installing them in the drums.
After the transmission is completely stripped down, the case needs to be cleaned. They are often coated with mud, dirt, grease, undercoating, and road tar. You can make a quick trip to a local carwash and easily remove most of the heavy debris. Spend some additional time with a screwdriver and small wire brush to loosen the stubborn dirt and grease. You may need some solvent to get the tar and undercoating off the case. Brake cleaner is a good solvent to help get oil and grease off the case, and it helps dry it as well. It leaves no oily residue behind, and also works quite well for cleaning internal components.
Several companies make heavyduty degreasers specifically designed for this purpose. Apply them before power washing or hand cleaning to help loosen thick greasy deposits from the case. Some of these products produce fumes so use them in a wellventilated area. Also wear a good pair of thick chemical-resistant gloves.
Bushing Removers and Installers
Automatic transmissions have quite a few bushings. Some are relatively easy to access, remove, and install. Others may be quite difficult to get to. If a bushing is readily accessible and flush with the top surface of the hole, removal and installation is relatively easy. Just about any suitable flat driver can be used to flush mount a bushing.
Some bushings are driven below flush or deep enough to be below a lube oil supply hole, for example. You must record your accurate measurements and carefully install these bushings to the same depth that they were originally at. This requires a bushing driver slightly smaller than the hole the bushing is driven into.
The lower case bushings may require a long extension to drive the new bushing properly into place. You need a large piece of solid steel pipe to help drive them in.
If a bushing is located in a blind hole, it may be difficult to remove. You can fabricate a tool with a sharp punch. Grind it on a slight angle so it can catch the edge of the bushing and drive it down. You then turn it sideways in the bore and easily pry it out with a large screwdriver.
Heli-Coils and Bolt Extractors
The TH400 is made of a castaluminum alloy. Steel fasteners are used to attach the various parts to them, such as the vacuum modulator, oil pan, etc. It is common to strip the threads out of the case in one or more places. This usually occurs with the oil pan attaching bolts. In some cases you can use a longer bolt to reach new threads. Another option is to move up to a bolt that is larger in diameter. The best repair is to install a thread insert into the case. This is accomplished by using a special tap supplied with the thread insert kit, then installing a steel threaded insert. This repair is stronger than the original material. Several companies make threaded insert kits. They come with a tap, several threaded inserts, and a special tool to install them.
Air tools are helpful in transmission rebuilding. They can considerably speed up the time needed to install and remove fasteners. Highpressure compressed air is also needed to bench test clutch packs and other components during the rebuilding process.
You need a blowgun with a long tip to access small holes in the case, the oil pump, and the clutch drums, in order to air test them. You can use a rubber tip or shop rag to help seal off the air to listen for any leakage at the newly installed seals. The blowgun can also be used to dry off parts and blow dirt off the case during cleaning.
Having compressed air is also a big advantage when cleaning gasket surfaces. You can use an air-driven disc grinder to remove stubborn gasket material and gasket sealers. The disks are available in several varieties, and the finer ones remove gasket material without damaging the aluminum surface where the gasket seals.
You can use impact guns to remove stubborn fasteners, such as those that attach the tail housing or rear transmission mount to the case. You can use a smaller, 3/8-inch impact gun to remove oil pan and valve body attaching bolts. An air ratchet comes in handy for these tasks as well, but is not as fast as an impact gun. However, if you use an impact gun to tighten fasteners, it should be set low enough to just snug them up. Apply any final torque to them with a torque wrench to minimize the possibility of stripping the threads out of the case.
Punches, Chisels and Files
A variety of punches and chisels are required for transmission work. They are used to “stake” parts in place, and to remove and install roll pins. Many bushings are difficult to remove without splitting them first, especially if they are in blind holes. You can grind a round punch at a slight angle and use it to catch the edge of the bushing. This drives it down on one side and facilitates removing it from the hole. Before removing any bushings, measure the depth they are set at so you can correctly install the new bushing.
You may need files to remove material from the manual shafts so they can be removed from the case. Most manual shafts have a lip that prevents them from sliding out of the case when you need to access the manual shaft seal. A few seconds of work with a small jeweler’s file removes any excess material and they slide right out of the case.
Lip Seal Installation Tools
Lip seals are used to apply pistons and servos to keep hydraulic fluid and pressure behind the piston. The lip on the seal is pushed tight against the bore it rides in, creating a positive seal. Lip seals can be somewhat difficult to install; there is usually an inner and outer seal on each piston.
Special seal installation tools are available for all transmission models. They keep the lip seals from getting torn during the installation process.
There is a slight factorymachined chamfer on the edge of each drum, to help with seal installation. Rather than buy special seal installation tools, you can use a feeler gauge or make your own seal installation tool from a small piece of copper or steel tubing and smooth wire. Insert a loop of wire into the open end of the tubing and crimp it tightly. Use the loop of wire to get the lip of the seal being installed past the chamfered edge without tearing it.
It takes some practice and patience to install lip-type seals without damaging them. The key here is to never force them into place or try to push the seal past a sharp lip without using some sort of installation tool. Air checking them, after the drum is assembled, ensures that you have not ripped or damaged the seals.
Friction Alignment Tool
The intermediate clutches in the TH400 transmission are located in the case. The back of the direct drum is splined to accommodate the intermediate friction plates. In order to install the direct drum, you must carefully align the friction in the center of the case. This allows the direct drum to drop fully into position. An alignment tool can be quickly fabricated by using an old outer race and welding a handle to it.
Another method that can be used here is to blow compressed air into the oil-supply passage for the intermediate clutch pack. This “bounces” the frictions and steels. Turning the direct drum while it is being installed and bouncing the intermediate clutch pack with compressed air aligns the frictions with the splines on the direct drum. It takes some patience when using this method, but it is an effective substitute for making an alignment tool for the clutch pack.
Oil Pump Band
lves. You must take them apart to access the pump gears, bushing, and seal. When they are ready to assembled, the two halves must be aligned or the pump will not fit back into the case. You can make a band from a large single hose clamp or by putting several large hose clamps together. Leave the pump bolts finger tight while tightening the band. Use a Phillips screwdriver or awl to align the bolt holes before tightening the pump bolts. It helps to drop the pump attaching bolts through the holes while the pump bolts are tightened. This ensures that the halves do not slip and get out of alignment. After the bolts are tightened, you should test fit the pump into the case and start all the pump attachment bolts by hand. Do not install any seals for the test fitting procedure.
Alignment studs are used to guide the oil pump into place during final installation. Use at least two studs. Place the gasket on the case first, and then thread the studs into the case by hand. They can be made from a couple of long bolts by cutting the ends off and grinding them to a point.
The TH400 transmission has two corresponding pump bolt holes tapped for a slide hammer. It is best to use two slide hammers to pull the pump evenly from the case. One hammer works, but you may have to alternate back and forth between the holes in the pump to effectively pull the pump from the case. You can thread a slide hammer into the pump to remove it from the case. Make sure to thread the slide hammer at least five full turns into the pump to avoid breaking off the tool or pulling out some of the threads.
The TH400 transmission doesn’t really require any special tools to effectively rebuild it. However, there are a few procedures that can be difficult to complete without them.One is the 2-3 accumulator located in the valve body. You can compress it with a large pair of channel locks in order to remove the E-clip on the pin. Compressing the accumulator for reassembly can be difficult. The accumulator spring is long enough that the sealing ring is outside of the bore before it is compressed. The accumulator piston must be compressed, and the metal ring fitted into the bore, at the same time. I modified a C-clamp to help with this procedure.
A large pair of channel locks works here as well. It may take a third pair of hands to align the accumulator piston with the pin and compress the metal ring at the same time.
A large pair of channel locks works here as well. It may take a third pair of hands to align the accumulator piston with the pin and compress the metal ring at the same time.
Written by Cliff Ruggles and Posted with Permission of CarTechBooks