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.
It’s almost certain that one or more bolt holes for the oil pan will require a Heli-Coil or other thread insert. They are easy to install, and the end result is stronger and holds better than the original material.
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.
Heli-Coil Installation
The Heli-Coil is very easy to install and takes only a little practice. The repair is stronger than the original material. In most cases, it is not necessary to drill out the old threads to start the Heli-Coil tap. Make sure that the tap is perpendicular to the mounting surface when it starts into the hole. Check the angle of the tap from several views to make sure that it is straight. Go slowly; a few drops of light oil or ATF helps the tap cut the threads cleanly. Tap deeply enough to accommodate the threaded insert or tap all the way to the bottom of the hole if it is a blind hole.
The most likely place a Heli-Coil installation is needed is on the oil pan rail. Most TH350s have had several fluid changes in their lifetime and at least one of the pan rail bolt holes will have stripped threads. In most cases, it is not necessary to drill out the hole to get the Heli-Coil tap started.
Tap the hole for the Heli-Coil deep to accommodate the threaded insert. Install the insert just below flush with the surface. Some bolt holes go all the way through; you may have to shorten the insert by a few coils so it is contained completely inside the hole and not sticking out on either end.
Once the insert is installed to the correct depth, remove the small bent end at the bottom and then remove the broken-off piece from the hole. The inserts have a small break on the first coil so the bottom of the insert breaks off easily without disturbing the rest of the coils.
Once the thread insert is correctly installed, it is stronger than the original aluminum material. Test fit a correct bolt into the hole; it should go all the way through the insert with minimal effort.
Blow out all the chips from the tapping process and insert the Heli-Coil insert with the tool provided. Be sure to insert the Heli-Coil insert about one thread below the flat surface so it has good purchase in the hole and doesn’t try to come out when the fastener is tightened. Use the provided tool to break off the bottom of the Heli-Coil; remove the small piece from the hole before you install the fastener.
Use high-pressure air to blow parts dry after cleaning. It can also be used to pressure test clutch pack operation. By applying air to the fluid supply holes, you can pressurize the passages with air, verify apply piston movement, and check that there is no leakage at the 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.
An angle grinder equipped with abrasive discs is one of the best inventions of modern times for cleaning off stubborn gaskets from machined surfaces. The discs are available in several different materials so they do not damage the soft aluminum surfaces.
Air tools can take a lot of time out of a transmission build. Even so, make sure they are going in the correct direction when removing bolts, which are easily stripped out in the soft aluminum cases.
Considering the age of many of these cases, and the fact they are aluminum, it’s usually best to avoid using an impact gun to tighten fasteners, even though it does save a lot of time. A spin-handle wrench is nearly as quick, and removes the possibility of stripping case threads. If, for any reason, an impact gun is used to tighten fasteners, it should be at its lowest setting; just enough to snug them up. Use a torque wrench for any final torque applied to minimize the possibility of stripping the threads in the case.
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.
Punches and chisels are handy items for transmission building. You may encounter roll pins that must be removed or installed; you may have to split a bushing to get it loose from its bore.
