The early e36 had a slightly different type of mounting bolt set up, with boots on the outside of the motor, it then switched to threaded holes in the motor housing. There's one bolt that is meant to be a pain in the ass to take in/out.

I've had a replacement starter motor ready to go on to my e36 for the last two years but I have put off doing the job loads of times!

The motor is the same across many generations and series:



And also many engine types:



Here is the later style: