Delco Starter Armature
 

I am rebuilding the original starter to my 71 gto project and have a question on the armature. The armature on the right in the first picture (1970548 GM part number) came out of my 71 gto starter and the one on the left came out of a parts Pontiac starter that appears to have been through a mass rebuilder at one time.

Should the armature commutator have grooves cut in it? The one to my gto feels fairly smooth and doesn't have very deep grooves. My first thought was that it was just wore out and needed grooves re-cut but it looks smooth even where the brushes were not riding. The brushes appeared to be the original Delco brushes and were wore completely out in this starter. The armature from the other starter has grooves cut in the commutator deep enough to catch a finger nail on.

Thanks

Your second photo of the armature end looks pretty done to me.

I use to know what the minimum OD was but I can’t recall what that dimension was.

I may still have it written down somewhere.

If I can find it I will post it .

Yes, the armature commutator insulation should be undercut or the brushes may not have good contact on the copper. The service manual directs one to do so in the electrical section.

I'm a little late to the party, but this is how it goes. The insulation between the commutator bars determines if it should be undercut or not. If the insulation is Bakelite, DO NOT undercut. If it is insulated with Mica, DO UNDERCUT. Undercutting removes a little of the insulating material between the commutator bars. The armature on the left looks to be for a low torque starter and the one on the right looks to be a high torque armature. Count the commutator bars. 23 bars is high torque and 27 bars is low torque. The high torque armature is wider than a low torque one. The high T armature won't fit in a low T case, but the low T armature will fit in a high T case, but even though it may spin it won't have any power.

Delco info: