vettjam

I have read that you should and should not undercut starter commutators. Which is correct? The shop I talked to about turning the commutator said they undercut generators but never starters. My service manual says to undercut. On the internet there seems to be strong opinions both ways.

Plasticman

I would recommend that if the comm needs substantial cutting, and that the comm surface cut will end up being below the level of the existing mica, then undercut first and then cut the comm surface, and then clean out the slots of debris.

If the comm is in decent shape, and you just want to clean it up, a pencil eraser (while the armature is turning in a lathe) works wonders.

However, reading a 74 Chevy manual, it states: Caution: Some starter motor models use a molded armature design and no attempt to undercut the insulation should be made or serious damage may result to the commutator. Undercutting reduces the bonding of the molding material which holds the commutator bars and since the molding material is softer than the copper bars, it is not necessary to undercut the material between the bars of the molded commutator.

I found the above note also in a 1970 GM Shop Overhaul Manual as well. I have a "general" service manual for the C2 that was printed in 1974, and it states the same above caution.

In other words, the shop may be correct, if he is seeing a molded commutator design on your armature. And since it may have been replaced at some point, it could certainly be a later armature with the molded comm.

Plasticman

stellar

If it has bakelite do not undercut. If it has mica do undercut.