December 23, 2015

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

Randall Schroeder

Tech support hotlines have been burning up for years with phone calls concerning rapid gear train failure in GM TH400 transmissions after routine rebuild or repair. It’s time to address the issues that cause this and reverse widespread confusion surrounding gear train setup on this and similar units. Today we will clear up the mystery in setting proper endplay for this transmission. If tolerances are not correctly set, costly comebacks can result. Understanding these basic checks on the TH400 will give you background knowledge in setting build procedures that will be used with similar gear sets, such as 4L80-E.

What causes these gear train failures? It often begins with a component replacement, whether changing a stock part or installing an upgrade. As parts are being replaced, taking time to set up endplay is necessary, but we run into trouble here because of incomplete OE reference information.

The TH400 (Figure 1) has four endplay tolerances (NOT to be confused with clutch clearances) that should be examined and corrected during repair. However, GM’s published rebuild procedure only references two of them! With two endplay tolerances being ignored, you can imagine why these meltdowns are occurring.

Figure 1

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).

Figure 4:

Rear Unit Selective Washers

Washer ThicknessI.D. NotchesI.D. Numeral
.074 - .078"None1
.082 - .086"Side 1 Tab2
.090 - .094"Side 2 Tabs3
.098 - .102"End 1 Tab4
.106 - .110"End 2 Tabs5
.114 - .118"End 3 Tabs6

It is vital to check the three needle bearing sets and their seats in the rear gear assembly, as wear here will affect clearances. If worn excessively, needle bearing sets are available in the aftermarket.

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 5Figure 6

Front Planetary to Rear Planetary Play

While there is no reference to this clearance in repair manuals, setting it correctly adds durability and life to the gear sets. The right endplay here will prevent excessive fore and aft movement under torque, preventing wear and quenching gear noise.

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

Forward Clutch Hub to Direct Clutch Drum Play

With the gear set assembly still secured in a vice, set the direct clutch drum on the assembly, ensuring it is fully seated on sun gear shaft (Figure 8).

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.

If clearance is greater than .020" and the thickest washer is being used, there are misassembled components or worn parts that were overlooked. WARNING: If there is improper clearance between these two assemblies, you will have aggressive wear and failure of the gear sets!

Front Endplay

Keep this simple and fast by leaving out the clutches of the Forward drum and holding back on installing the direct drum into the case until you have checked the 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

The washer between the pump and Forward drum is selectable; to help gain tighter tolerances, there are other thicknesses available in the aftermarket (Figure 10). When adjusting final front endplay, set it .002" to .005" looser than the rear play. For example, if rear endplay was to have measured .010", you would set the front play around .012" to .015". This will allow for forward movement of the rear gear set while avoiding making the front too tight.

Figure 10:

Front Unit Thrust Washers

Washer ThicknessI.D. NumberColor Code*
.060 - .064"0Yellow
.071 - .075"1Blue
.082 - .086"2Red
.093 - .097"3Brown
.104 - .108"4Green
.115 - .119"5Black
.126 - .130"6Purple
*Oil soaked washers may discolor. Measure actual thickness.

Set-up is as easy as one, two, three, and four:

  1. 1. Rear Endplay: .007" to .019"
2. Planetary Clearances: .008" to .015"
3. Forward Clutch Hub to Direct Drum: .008" to .015"
4. Front Endplay: .002" to .005" looser than rear endplay

There is an old saying: “I don’t have time to play.” But investing a few minutes for “playtime” is vital to protect against comebacks, to ensure maximum longevity for your rebuild, and to provide ultimate satisfaction for your customers.

Randall Schroeder is a Sonnax technical specialist and member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.

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