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Servicing or reinstalling the auxiliary section on an Eaton Fuller heavy-duty transmission? Follow this step-by-step professional workshop procedure to select the correct M40080 retaining plate thickness (.100" or .125"), lock auxiliary countershafts in parallel alignment, verify gear timing marks, and slide the auxiliary housing into place smoothly without gear tooth binding or bearing damage.

Why Unsupported Countershafts Cause Catastrophic Transmission Failures

On Eaton Fuller commercial truck transmissions, proper gear mesh between the mainshaft and twin auxiliary countershafts depends on precise timing mark alignment. When assembling the auxiliary section, both countershafts must remain centered and parallel with the mainshaft center axis. Technicians who attempt to install auxiliary housings without dedicated countershaft retaining tools quickly encounter misalignment. As the heavy auxiliary case is pushed onto the main transmission case, gravity pulls the unsecured countershafts downward. The countershaft gears strike the mainshaft gears at an angle, chipping gear teeth and pinching taper roller bearing races. Furthermore, off-center shaft movement causes the gear timing teeth to jump mesh, misaligning the timing marks. A mis-timed transmission produces destructive gear vibration, rapid bearing disintegration, and complete transmission failure shortly after being put back into highway service. Utilizing the 8-pound M40080 Auxiliary Countershaft Retaining Kit completely eliminates these rebuild risks. By bolting the correct thickness retaining plate (.100" or .125") directly to the rear housing, the countershafts are rigidly locked in parallel alignment, allowing the auxiliary case to slide on effortlessly with 100% accurate timing engagement.

The 5-Step Eaton Fuller Auxiliary Countershaft Protocol

1. Clean Housing Face & Inspect Countershaft Bearings: Thoroughly clean the rear face of the transmission housing and auxiliary case mating flange using solvent. Inspect taper roller bearings and countershaft ends for wear, burrs, or debris.
2. Select Correct M40080 Plate Thickness (.100" or .125"): Check your Eaton Fuller manual specification for housing series clearance. Select either the **.100"** or **.125"** M40080 retaining plate that provides snug, flush seating against the countershaft rear bearing race face.
3. Mount M40080 Retaining Plates & Lock Countershafts: Position the M40080 retaining plate over the auxiliary countershaft ends. Thread mounting cap screws through the plate into the housing bolt holes. Torque cap screws evenly to clamp both countershafts firmly in parallel alignment.
4. Verify Auxiliary Gear Timing Mark Alignment: Paint or mark the timing teeth on the mainshaft drive gear and both auxiliary countershaft gears per Eaton Fuller service manual instructions. Rotate gears to confirm timing marks align perfectly centered before mounting the case.
5. Slide Auxiliary Housing On & Remove Retaining Plate: Carefully guide the auxiliary housing over the locked countershafts and mainshaft until fully seated against the main transmission case. Torque housing bolts to OEM spec, then unbolt and remove the M40080 retaining plates before final rear cover installation.

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