Playing with Transmissions: Setting Total Endplay to Prevent TH400 Gear Train Failure

The following four areas should be checked and corrected; the last two are not referenced in OEM material.

• Rear Endplay – Sets up from center of case to top of rear case selective plate.

• Front Endplay – Sets up between top washer of Forward clutch hub and selective pump washer.

• Front to Rear Planetary Endplay – Important with changing torque load.

• Forward Clutch Hub to Direct Clutch Drum Endplay – Measured between the Forward clutch hub and direct clutch drum as they are assembled on their respective shafts. Very important, despite being omitted from OEM reference.

Let’s look at these more closely.

Rear Endplay

The components involved with rear endplay are (from center of case back):

• Center support along with top sun gear needle bearing set
• Sun gear/direct clutch drum shaft
• Sun gear
• Bottom sun gear needle bearing set to top of rear planetary ring gear
• Bottom of rear planetary ring gear to top of output shaft needle bearing set
• Output shaft and bottom of output shaft (four-tab washer)
• Selective washer at bottom of case

The selective washer at the bottom of the case has tabs to lock into and secure against the rear of the case; these tabs also identify washer thickness.

Checking rear play is easier to manage if the planetary assemblies are left off (Figure 2).

Figure 2

The rear gear train must be installed into the case and secured with a snap ring on the center support. Mount a dial indicator at the rear of the case to measure the movement of the output shaft, ensuring it moves freely in and out. With the shaft pushed against the washers, zero the dial indicator and move the assembly in the opposite direction to get your measurement (Figure 3).

Figure 3

Rear endplay needs to be .007" to .019". If out of specification, the selective washer must be replaced with an appropriate washer (Figure 4).

During rear gear train assembly, ensure the sun gear shaft and sun gear are both assembled properly; if installed incorrectly, it will affect direct clutch drum settings. Proper assembly will leave the long splined end of the shaft mated to the flat side of the sun gear, not the beveled side (Figures 5 & 6). If there are no alignment splines machined into the gear and shaft, ensure the lube hole in the shaft lines up with those on the gear.

Figure 5	Figure 6

Front Planetary to Rear Planetary Play

Assemble the rear gear sets as you would to install them in the case, including the center support. Use a suitable holding fixture to secure the assembled gear set upright (a vice works well). It is virtually impossible to get a dial indicator set up here, so we are going by “experienced feel” in this case. Lift up and down on the top planetary to center support (Figure 7). If excessive play is discovered, this clearance can be adjusted by using a selective four-tab washer between the gear sets. Set this up between about .008" and .015", with .010" being the sweet spot.

Figure 7

Figure 8

Install the washer that sits between the hub and the drum, then set hub on the support shaft and hold it tight. Lift up on the drum to check for clearance between these two rotating pieces; we’re looking for .008" to .015" here. If no clearance is found, use a thinner washer. If excessive clearance is found, use a thicker washer, available in the aftermarket.

Front Endplay

With the back of the case assembled up to the intermediate clutches (secured with snap ring), set the Forward hub in position. Install the washer that goes between the top of this hub and the Forward clutch drum. Rest the Forward drum (empty) on top of the hub. Place the pump gasket in the case so we don’t overlook its thickness. Leave the rubber seal ring off the outer side of the pump and set the pump in place (secure with one bolt, finger-tight). Use your dial indicator to measure the up and down movement of the input shaft (Figure 9).

Figure 9