Turbo 400 Rebuilding Tips and Tricks

The other internal element that greatly benefits from a light block sanding is the cover for the Reverse apply piston. These stamped steel covers tend to warp, causing low pressure for the Reverse band servo.

Most TH400 transmissions came from the factory with the forward sprag shown here. This design is prone to failure because of its intricate internal spring arrangement with few rollers to distribute torque loads over the drum.

Quality Transmission replaced the factory sprag with the superior 32-element design shown here. This part comes from Transtar and requires a special drum that virtually eliminates any issues associated with breaking the sprag-a common failure point in the TH400.

This photo illustrates the difference between the stock springs and the heavy-duty springs that come with the Transgo shift kit. The shorter spring on the left is constructed from thicker, higher-quality wire and prevents the clutches from applying centrifugally at high rpm.

Cast aluminum versus stamped steel: when it comes to the TH400's clutch apply pistons, aluminum is the clear winner. Here's why: the stamped steel unit consists of two individual pieces that press together. Sometimes this press fit loosens over time, and fluid pressure loss is the unavoidable result. The cast piston on the left is a single part, and leaves no room for a seal to leak or gap to develop in the first place. Another argument in favor of the aluminum-type piston is that of rigidity. The aluminum piston will flex much less than a steel unit, thus preserving the outer diameter seal and sealing surface from wear. When changing out pistons, builders must take note that a check ball exists in either the piston or the forward/direct drum; without one, you are asking for trouble.

Transgo provides improved Teflon seals for the center support unit. These seals replace steel-type seals and are far superior at sealing the direct clutch circuit.

Ideally, you want to fit six frictions inside the direct clutch apply drum on the TH400. To achieve this, a transmission builder must select the thinnest combination of steel discs and friction elements. The more you fit, the longer the life span of the clutch material will be. However, if you install too many clutch packs, you will not have enough endplay and this can cause a harsh reverse engagement. Ideally, you want 0.004 to 0.006 inch of endplay in the direct clutch pack.

This shot shows the three available output shafts for the TH400 automatic, the shortest of which is the one required for mating the TH400 to the popular Atlas II or Off-Road Design 203/305 Doubler kit. Our plan is to use the Doubler, which you will be able to read all about in next month's issue.

Also included in the Transgo shift kit is this shift valve along with a cup plug, which enables compression braking in all forward gears. This option is imperative for anyone who uses low-range when on the trail, or for those who tow heavy loads over mountain passes. Essentially, this part lets the engine slow the vehicle when you let off the gas in gear or downshift to slow the vehicle. The one on the left is the new Transgo unit.

The TH400 features two filters inside that help remove debris from the ATF. The small screen filter shown here eliminates contaminants from entering the governor unit. This is super important to replace when rebuilding because metallic debris can jam the governor unit and cause the transmission to shift erratically.

Every transmission rebuild should include a new filter. The one shown here is for the deep pan commonly found in heavy-duty applications such as 3/4- and 1-ton pickups.

The torque converter we used for this build came from PDQ Transmission Parts and features a stock stall speed and anti-balloon plate. This converter also uses a heavy-duty enclosed Allison bearing and features furnace-brazed turbine impeller and vane plates for long-lasting durability. FW