On 10/28/2015, Kurt Bangert wrote:
"A quick note on slow starter rotation". My MGA was cranking slowly, and after looking at all else took it apart and the brushes were worn completely concave. Wanting to get the car going right then and without new brushes available handily, I filed the sides of the brushes at a 45d angle on each side leaving about 1/4" in the center (filed flat) and replaced them. Worked perfectly and has been going fine for the last two years. One of these days I'll get new ones put in place. Thought this might help if someone's scratching their head after all the other checks.


M35G (early MGA)M35J (late model MGA)
There are two different types of starter motor commutators and brushes. The early one has radial brushes bearing against the side of the commutator. These are the brushes that will wear to a concave end shape. The later one has pie shape brushes (commonly copper brushes) that bear against the flat end of the commutator. As these wear they stay pretty much flat. Starter motor brushes are often copper rather than carbon, and the copper ones should last indefinitely (almost forever). Starter motors usually run for only a few seconds at a time, so if properly treated may last a life time.