Starter rebuild

The rebuilding of starter motors is a less common practice today than it used to be, but it is still performed. The procedure outlined here references that for a Delco-Remy 42MT, which is one of the most commonly used in the truck industry. As usual, you should consult the OEM service literature when reconditioning starter motors in the field.


1. Scribe the starter housing so that the locations of the solenoid, lever housing, nose plate, and endplate are indicated for reassembly.

2. Disconnect the field coil connector from the solenoid to motor terminal and the solenoid ground lead where applicable.

3. Remove the brush inspection caps and then remove the brush lead caps. This separates the field leads from the brush holders.

4. Remove the solenoid and then remove the through bolts, separating the commutator end cap from the field frame body. Separate the brush plate assembly from the field frame body.

5. Next, separate the nose housing and lever housing. Remove the pinion stop.

6. Remove the clutch assembly retainer and snapring from the armature shaft by driving the collar toward the armature core.

7. Separate the clutch assembly from the armature shaft—and the armature from the lever housing. Press the bearings from both the gear housing and the drive housing using a mandrel.

8. Clean all of the disassembled components with a solvent with no residual oil and a firm bristle brush. Allow the components to air dry. If the commutator is dirty, clean it with #100 or finer sandpaper. If the commutator cannot be cleaned the armature assembly should be replaced.


1. Check the brushes for wear and replace if necessary: The brush contact face should have maximum contact with the commutator. The brush holders should be checked for binding and the springs visually inspected. If the springs are discolored or distorted, replace them.

2. Inspect the armature assembly. Check for loose commutator connections. These can be resoldered to the riser bars, but it is not recommended. Test the armature for shorts, grounds, and opens.

3. A growler is used for testing shorts. Place the armature on the growler and hold a steel strip on the armature; it will vibrate over a short.

4. Using a test lamp, check for armature grounds; one lead should be placed on the commutator and the other on the armature shaft. If the test lamp illuminates, the armature is grounded and should be replaced.

5. Locate opens by checking for continuity any place that conductors are joined to the commutator. Either resolder connections or replace the armature if there are opens.

6. Test the field coils for grounds by using a 110- volt test lamp and connecting one lead to the field frame and the other to the field connector. If the lamp illuminates, the field coils are grounded.

7. Check the field coils for opens with the testlight: If it does not illuminate, the field coil is open. If you have to replace the field coils, use a pole shoe spreader and pole screw screwdriver.

8. Check out the bearings by rotating and noting any roughness.


1. Install the field coils in the housing. Immerse the bearings and both tangent wicks in engine oil prior to installation.

2. Press the bearings into the housing using a mandrel.

3. Next, install the oil seal in the lever housing followed by the field frame. Coat the spacer washer with the appropriate lubricant and fit it to the armature shaft. Then insert the armature into the field frame, making sure you align the scribe marks made on disassembly, and leave the commutator exposed.

4. Run the through bolts through the assembly and insert the brush inspection caps.

5. Now install the clutch assembly onto the armature shaft and install the solenoid plunger and shift lever mechanism. Ensure that the shift lever assembly operates freely. Place the clutch assembly retainer onto the armature shaft, ensuring that the cupped surface faces the snapring groove. Then slide the snapring into position.

6. Install the solenoid and field coil connection. Install the ground lead securely. Finally, check pinion clearance as outlined in the next section.


1. Disconnect the field coil connector (the M terminal) from the solenoid motor terminal and insulate it. Connect a battery (same voltage as solenoid) from the solenoid S terminal to the solenoid housing.

2. Flash a jumper lead from the solenoid Mtr terminal and the ground return terminal; this should kick the starter shift pinion into cranking position, where it will remain while the battery is connected.

3. Apply a little pressure on the pinion, forcing it back toward the commutator, and measure clearance between the pinion and nose housing as shown in Figure 9–23. The specification should be typically around 0.40 inch but check the Delco-Remy data.

4. To adjust pinion clearance, remove the plug on the rear of the solenoid and turn the shaft nut that connects the shift lever to the solenoid as shown in Figure 9–23.

FIGURE 9–23 Pinion clearance on a 42MT starter motor: Measure with feeler gauges—adjust at shaft nut if required.

FIGURE 9–23 Pinion clearance on a 42MT starter
motor: Measure with feeler gauges—adjust at shaft nut
if required.

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