This chapter is one of the most important in this book.
Transmission rebuilding is not overly difficult, but it does involve attention to detail, and exact precision in every area. This does not mean that completely and correctly rebuilding an automatic transmission is beyond the scope and capabilities of the average automotive enthusiast. Automatic transmissions have always had a mystique about them that keeps away even the most skilled automotive technicians. There is no need to carry your TH350 off to a shop or to someone with more experience. A complete and correct rebuild can be accomplished in a small shop or garage. Anyone taking on the task simply needs to be armed with good information, a few tools that I describe, and good parts.
This Tech Tip is From the Full Book, GM TURBO 350 TRANSMISSIONS: HOW TO REBUILD AND MODIFY. For a comprehensive guide on this entire subject you can visit this link:
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At the very top of the list of necessary items is a clean work area. An automatic transmission is a hydraulic or fluid-drive system. All parts must be kept dirt free and working smoothly for correct function. The person rebuilding the unit is essentially returning the main components to the same location, while, as necessary, replacing parts that wear out.
My goal is to outline the tools and equipment that help get the job done, showing tools that are often already part of the average automotive enthusiast’s toolbox, and tools that can be obtained easily, modified, or fabricated to get a particular task accomplished, without having to spend a lot of money buying them.
It is true that a few special tools may be required for certain operations on several of the units I cover.
The good news is that the majority of the tools required are simply plain old hand tools, or tools that are easily fabricated if a procedure requires something not typically found in the average toolbox.
The biggest hurdle to overcome when rebuilding TH350 transmission is removing the low/reverse piston’s snap ring and spring cage from the case. That part of the build will require some sort of spring compressor. One can be fabricated easily from a long threaded bolt or piece of threaded rod, a couple of pieces of flat steel, and a nut.
Compress the spring cages on the high and forward drums. You can use the same type of tool for the direct drum, but the forward drum becomes more of a challenge because of its attached shaft. This requires a press or spring compressor that holds the direct drum in position and allows a compressor to center over the spring cage to compress. Then you can remove the snap ring.
First and foremost on your list should be safety equipment. Automatic transmissions use pressurized fluid. During the rebuild, you come into contact with transmission ﬂuid and cleaning solvents. You will also be using compressed air for drying parts as well as air pressure for testing clutch packs, servos, etc.
Eye protection is a must anytime compressed air is used. Nothing is worse than getting high-detergent transmission fluid into your eyes, or a piece of dirt or debris when you are blowing off parts.
Nitrile gloves are a good idea for keeping solvents and transmission fluid off your hands. They also provide some protection against cuts from 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 and chemical-resistant gloves also provide protection. Wear them while cleaning the transmission case with heavy-duty degreasers or other harsh chemicals.
Hearing protection is also a very good idea. During the rebuild, compressed air is used to clean parts and dry them. For example, when high-pressure air is blown into the case passages, it often creates a high-pitched sound that can damage your hearing.
Sockets and Wrenches
General Motors used both metric and SAE fasteners throughout the years of production. Later units have metric fasteners; some units have both SAE and metric on the same unit. A complete socket set with 1/4-, 3/8-, and 1/2-inch-drive sockets is needed. Make sure to have extensions for each drive.
An air ratchet and an impact gun speed up the rebuilding process considerably. However, neither should be used to tighten fasteners because most go into the aluminum case, which is soft enough that the threads may strip out if excessive torque is used.
A torque wrench is necessary, as each fastener size has a torque value based on its diameter and hardness. The long handle on a torque wrench makes it very easy to over-tighten a bolt and strip the threads out of the case. This is easiest to do with the smaller fasteners that hold the valve body in place. Be careful and follow torque specs precisely.
In addition to a complete socket set, some hand wrenches are required. I especially like the new styles that have the ratcheting feature on the boxed end. They speed up the time typically required to tighten or remove a fastener that’s not accessible with a socket. Flare nut or tubing wrenches are made specifically for accessing the nuts that hold steel lines in place.
These wrenches have additional material and increased integrity to loosen stubborn flare nuts that hold the steel cooling lines to the case. They allow the wrench to slip over the tube, and still get a good purchase on the flare nut to remove it easily.
Snap Ring Pliers
Several varieties of snap ring pliers will be needed, including some that will be modified for a particular purpose. On the TH350 and 4L60 transmissions specifically, the output shaft uses a small snap ring for retention, which is difficult to access. Pry it out carefully with a very small screwdriver, or use modified ring expanding pliers, which are easier.
Pliers with removable tips are also available; one pair can cover a broad range of applications. Some snap ring pliers are also designed to be convertible, and can be modified in seconds to remove either inner or outer snap rings.
A variety of screwdrivers come in handy during the rebuild. Large flat-blade screwdrivers can be used to remove the snap rings in the transmission cases and also to reposition them. And, Phillips screwdrivers also double as alignment tools when putting the pump back on the case. A few minutes with a torch can quickly take a couple of small flat-blade screwdrivers and turn them into friction and steel removal toolsBy bending over the ends, they can be used to reach inside clutch drums or into the transmission case to pull steels and frictions out.
A hook and pick set is handy for removing lip seals and lifting steel and friction plates from the clutch drums. An awl is handy for transmission rebuilding. It can be used to help line up bolt holes with gaskets and separator plates when installing the valve body, and also to remove the snap ring that retains the 1-2 accu-mulator in the TH350 case.
Transmission rebuilding involves precision and attention to detail; many items in the assembly must be measured for specification. A dial indicator is a nice addition to the arsenal for transmission builds. It can be used to check input or output shaft endplay before and after the unit is assembled. Because you may have selective shims available to set the shaft endplay, it’s nice to be able to measure it accurately. The other end of the dial indicator has a depth gauge, which is another handy tool. It tells you how far down in a drum a particular bushing was installed. You need to know this measurement before driving it out for replacement.
A standard caliper is a great tool to measure friction and steel plate thickness, as well as the total thickness of clutch packs. It can also be used to measure the bushings’ diameter (both outside and inside), as well as the thickness of apply pistons, steel plates, and frictions.
A standard 1-inch micrometer works equally as well for measuring frictions and steel plates, as well as the total thickness of a clutch pack.
You should also add a feeler gauge to your list. They are used to check clutch pack endplay in the forward and direct drums. A feeler gauge is also used to install the apply pistons in the forward, direct, and intermediate drums.
Several of the units covered in this book require a piston spring compressor inside the case. The TH350 and 4L60, specifically, use a low/reverse piston in the lower portion of the case. A snap ring holds the spring cage in; it must be compressed in place for removal.
You can use these to compress spring cages on other components, provided those pieces have a center hole. For drums without a center hole, the component must be held stationary and some sort of spring compressor used to push down on the spring cage to facilitate the removal of the snap ring.
A larger tabletop bench-mounted spring compressor is needed for building clutch drums. A shop press works equally as well, although this type of press tends to be much more difficult to set up and a lot slower to use.
Many years ago, I made a spring compressor from two pieces of ﬂat steel bar left over from other projects in about 5 minutes. It has served me well for decades and the total investment in the part was only a few dollars. Similar tools can be fabricated from steel flat plate and longer pieces of threaded rod. It mounts easily in a large shop vise for clutch drums with a protruding shaft. If necessary, a hole can be drilled through the workbench to accommodate these drums, which allows the tool to be used on a flat work surface if a large vise is not available. If you don’t want to build your own, spring compressors are also available through most commercial tool supply stores.
Case Holding Fixtures
While certainly not required, a transmission holding fixture, if available, makes the build go much easier. However, being able to turn the transmission in any direction and lock it there really helps with the disassembly and assembly process. Several different types are available.
General Motors was kind enough to 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. Because 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 that bolt 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 a holding fixture of any type is to be used, it needs to be securely attached to a heavyduty table or workbench. The workbench may need to be counterbalanced to offset the weight of the transmission. A simple heavy-duty workbench can be easily constructed from a couple of 4 x 8-foot sheets of plywood, 4 x 4s, several 2 x 6s and some good wood screws. The benches I use in the shop have storage space under them. The weight of the parts on the lower shelf provides enough stability so that the weight of the transmission doesn’t cause the table to fall over. The 4 x 8 work surface provides plenty of room to spread out all the parts removed from the transmission.
Roll-around carts also make a great place to lay out rebuild kits, bushing sets, thrust washers, and other items used to rebuild transmissions. They can be pulled right up to the work area without mixing up the new parts with used ones, and then pushed away when cleaning off and blowing parts dry with compressed air.
A shop vise, especially a large one, can be a nice addition to your list of tools for your transmission rebuild. A set of soft jaws can be added to hold shafts without damaging them. You can also use it to hold the valve body when you need a third set of hands to keep it stationary for removing valves, springs, etc.
Although not part of the list of tools for most folks, having a small lathe or access to one helps if you are rebuilding a “max-effort” TH350 transmission. It is also especially useful if you want to increase friction capacity to one of the drums or custom set the endplay for a particular clutch pack by removing some material from the apply piston. Most TH350s show up with a four-clutch-pack direct drum. It is very easily increased to five frictions by removing some material from the apply piston. You can also use a lathe to remove material from the low/reverse apply piston, or forward drum apply piston. You can also use it if you are working with a light-duty TH350 and want to increase the clutch capacity in those drums.
Pans for Small Parts
Large commercial-grade cookie sheets are great for laying out 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 off onto the floor. Frictions and bands need to be soaked in clean automatic transmission fluid (ATF) for at least 15 minutes prior to assembly. A one-gallon ice cream pail with about a quart of ATF works well for this; they generally have a lid to keep out dirt and debris. ATF also works well for lubricating Torrington bearings, and dipping apply pistons prior to installation in the drums.
Once the transmission is completely stripped down, the case needs to be cleaned. Cases are often coated with mud, dirt, grease, undercoating, and road tar. A quick trip to a local carwash gets rid of most of the heavy debris. Some additional time spent with a screwdriver and small wire brush loosens the stubborn dirt and grease. Some solvent, such as brake cleaner, may be required to get the tar and undercoating off of the case. Brake cleaner also helps to dry the case; it doesn’t leave an oily residue. It also works quite well for cleaning internal components.
Several companies make heavy-duty degreasers specifically designed for this purpose. Applying these cleaners before power washing or hand cleaning helps to loosen heavy greasy deposits from the case. Some of these products produce fumes so they should be used only in a well-ventilated area. Also use a good pair of thick chemical-resistant gloves.
Bushing Removers and Installers
Automatic transmissions have numerous bushings. Some are relatively easy to access for removal and installation, while others may be quite difficult to get to. If a bushing is readily accessible and driven in flush with the top surface of its 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 in below flush. They may be, for example, deep enough that they are below the lube oil supply hole. Careful measurement and care must be used to install these bushings to the same depth at which they were originally installed. This requires a bushing driver slightly smaller than the hole into which the bushing is driven. If a bushing is located in a blind hole, it may be difficult to remove. A sharp punch ground on a slight angle can be fabricated to catch the edge of the bushing and drive it down. The bushing turns sideways in the bore and can then be pried out with a large screwdriver.
The lower case bushings on TH350s and TH400s may require a long extension to drive the new bushing in place. You need a large piece of solid steel pipe to help drive them in. The front output shaft bushing in the TH350 transmission can be effectively removed with a large tap. Tap the bushing and continue to turn the tap until it hits the bottom of the hole; it pushes the bushing out of the output shaft. This bushing is difficult to remove by any other method.
Use a large punch ground at an angle (as described above) to remove this bushing. You need a shop vise with soft jaws to effectively hold the output shaft. With a hammer and angleground punch, drive down on the edge of the bushing. Locate the seam if possible, and collapse the bushing; it can then be removed from the output shaft. Use care for this procedure to avoid damaging the inside of the bore where the bushing is located.
Heli-Coils and Bolt Extractors
The transmissions that I discuss here are cast aluminum alloy. Steel fasteners attach the various parts, such as the vacuum modulator and oil pan. It is not uncommon 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, a longer bolt can be used to reach new threads. Another option is to use a larger-diameter bolt.
However, the best repair is to install a thread insert into the case.
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 speed up the time considerably that is needed to install and remove fasteners.High-pressure compressed air is also necessary to bench-test clutch packs and other components during the rebuilding process.
A blowgun with a long tip is essential to access small holes in the case, oil pump and clutch drums to air test them. A rubber tip or shop rag can be used 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 of the case during cleaning.
Another big advantage of having compressed air available is cleaning gasket surfaces. You can equip a high-speed air grinder with abrasive discs to remove stubborn gasket material and gasket sealers. The discs are available in several varieties; the finer ones remove gasket material and will not damage the aluminum surface where the gasket seals.
An air ratchet also comes in handy for these tasks, but it is not as fast as an impact gun. Impact guns can be used to remove stubborn fasteners such as those that attach the tail housing or rear transmission mount to the case. A smaller 3/8-inch impact gun can be used to remove oil pan and valve body attaching bolts.
Punches, Chisels and Files
A variety of punches and chisels are required for transmission work. They are used to “stake” parts in place, as well as to remove and install roll pins. Many bushings are difficult to remove without splitting them first, especially if they are in blind holes.
A round punch can be ground at a slight angle and used to catch the edge of the bushing. This drives it down on one side and facilitates removing it from its hole. Before removing any bushings, measure the depth at which they’re set so you can correctly install the new bushing.
Files may be required 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 (to access the manual shaft seal). A few seconds with a small jeweler’s file removes any excess material so that the shaft will slide right out of the case.
Lip Seal Installation Tools
Lip seals are used on apply pistons and servos to keep hydraulic fluid and pressure behind the piston. The lip on the seal is pushed tight against the bore in which it rides to create a positive seal. Lip seals can be somewhat difficult to install because you generally have to work with both an inner and outer seal for each piston.
Seals, O-rings, and Teflon seals can be difficult to remove from apply pistons and accumulators. A razor blade, razor knife, or a small pocket-knife with a very sharp blade comes in handy here. Cutting the rings off the apply piston or accumulator is preferred instead of prying them off with a small screwdriver. The aluminum is less likely to be damaged, which could, in turn, have cut or damaged the new seal.
Special seal installation tools are available for all transmission models. They prevent the lip seals from tearing during the installation process. The factory machines a slight chamfer on the edge of each drum to help with seal installation. Instead of buying special seal installation tools, you can use a feeler gauge or make your own seal installation tool from a piece of small copper or steel tubing and smooth wire. Insert a loop of wire into the open end of the tubing and crimp it tightly. The loop of wire is used to get the lip of the seal that you are installing past the chamfered edge without tearing it. Inconel or music wire works best because it is more rigid than soft metal wire.
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 them past a sharp lip without using some sort of installation tool. Air checking the seal once the drum is assembled ensures that you have not ripped or damaged the seals. Air checking simply involves applying compressed air to the supply passage in the drum where pressurized transmission fluid would normally move the apply piston. Some drums can be pressure tested on the workbench before installation. Alternatively, you can place the transmission oil pump in a soft-jawed vise with the drums and sealing rings in place, and then apply pressurized air through the appropriate supply holes at the pump’s mounting surface.
Oil Pump Band
Oil pumps are designed as two halves. You have to take them apart to access the pump gears, bushing, and seal. When they are ready for assembly, you must align the two halves or the pump will not fit back into the case. Make a band from a large hose clamp or put several large hose clamps together. The pump’s bolts should remain finger tight while the band is tightened. Use a Phillips screwdriver or awl to align the bolt holes and then drop the attaching bolts through the holes while the pump’s bolts are tightened. This ensures that the halves do not slip out of alignment. Once the pump’s bolts are tightened, test fit the pump into the case, and then start all of the pump’s attaching bolts by hand. Do not install any seals for the test fitting procedure.
Alignment studs guide the oil pump into place during final installation. Use at least two studs. Place the gasket on the case first, then hand thread the studs into the case. To make a couple of studs, simple cut the ends off a couple of long bolts and grind them to a point.
The TH350 and TH400 transmissions have 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. You can use only one hammer, but you may have to alternate back and forth between the holes in the pump to pull the pump from the case effectively. Thread a slide hammer into the pump to remove the pump from the case. Make sure to thread the slide hammer at least five full turns into the pump to avoid breaking the tool off or pulling out some of the threads.
Rebuilding the TH350 transmission effectively doesn’t require special tools. If you purchased all the special tools that you could use for a TH350 rebuild, you would quickly offset any savings from doing the rebuild yourself. For that reason, throughout this and other chapters, I provide alternatives: How you can use an alternative procedure, substitute a common tool for a special tool, or use a different procedure to accomplish a particular task for which a special tool would normally be used.
Even so, there are a few procedures that can be difficult to complete without a special tool. Installing the TH350 1-2 accumulator cover is one of them. The accumulator uses a very strong spring under the cover, which is very difficult to push into position without dislodging the O-ring under it. If the O-ring is dislodged, you’ll have a huge oil leak when the transmission is placed into service. You can use a large pry bar to easily compress the accumulator cover to remove or install the snap ring. You need a long 5/16-inch bolt, which is inserted into one of the oil pan bolt holes. This makes a perfect location for levering the pry bar to compress the accumulator cover.
The accumulators in the valve body of the TH350 and TH400 can be difficult to remove. A modified C-clamp effectively compresses the accumulator piston so you can remove the retaining clip. Large channel lock pliers also work for this purpose. Gently clamping the valve body in a shop vise with soft jaws or wooden blocks makes the job even easier.
Written by Cliff Ruggles and Posted with Permission of CarTechBooks