BMW 528i won't start when hot

Spec: BMW 528i 1996. Cold start is absolutely normal. When the motor is hot, the starter motor does not turn at all, but it sounds like the solenoid engages. I looked at the starter from below the car and it seems that there may be a mounting bolt missing (I only see two). If there are meant to be more than two mounting bolts, then the problem may be due to warping from the heat; if not....does anyone have any suggestions? Thanks.

Sounds like it needs brushes.

The brushes are made of carbon.

It, carbon, is used since it does not grow when heated.
Carbon will shrink when heated. If the length is shortened by usage, and it is normal for that to happen.
You get the symptoms you are reporting.

Later,

yes, it sounds like a starter motor problem and there are only 2 mounting bolts. you'll want to check the engine ground strap and be sure the battery terminals and cables are clean and tight.
if these ARE good then i'd recommend a BOSCH reman. unit as opposed to having it 'repaired' or 'rebuilt'..........reliable electrical shops that are equipped or can do this properly are hard to come by in the U.S.