Fitting a new commutator is a workshop job rather than a roadside one, but for an auto electrician or rewind shop it is routine. This is what it involves, so you know what you are buying and what else you will need.
1. Strip and record
Remove the armature from the starter. Before anything comes off, photograph the commutator end and note which winding leads go to which bar. On a lap winding the pattern is regular, but a photo saves an hour later.
2. Disconnect the windings
Unsolder or cut the winding leads from the risers. On welded commutators the leads are cut flush and the ends cleaned back to bright copper. Keep the leads as long as possible.
3. Press off the old commutator
Commutators are an interference fit on the shaft and come off with a press or a puller, supporting the lamination stack and not the shaft end. Note the position on the shaft: the new one goes back to the same place so the brushes land in the middle of the brush surface.
4. Fit the new commutator
Check the bore of the new part against the shaft; it should need pressing, not tapping. Heat helps on larger units. Press it to the recorded position, keeping it square, and check the run-out with a dial gauge before going further.
5. Connect the windings
Lay the leads into the hooks or slots in the recorded pattern. Soldering with a high-melting-point solder is the traditional method; resistance brazing or TIG is used on heavier units and where the starter runs hot. Every joint must be sound: a dry joint is a burnt bar in six months.
6. Skim and undercut
Skim the brush surface in a lathe to true it to the shaft, taking the minimum off. Undercut the mica between segments to roughly 0.5 to 0.8 mm below the copper, then chamfer the bar edges lightly and polish. Blow out all copper dust from the slots.
7. Test
Growler test for shorts, bar-to-bar resistance for opens and high-resistance joints, and an insulation test from the bars to the shaft. Balance if the armature is large or runs fast. Then refit with new brushes; old brushes bedded to the old surface will spark on a new one.
Do it yourself, or use a rewind shop?
If you have a press, a lathe and a growler, this is a two-hour job. If not, most towns have a rewind shop that will fit a commutator you supply. We sell to rewinders and to workshops doing their own, and a trade account is available for either.