This article discusses the ideal interval between drive belt replacements for the E28 and E34 with the M20 engine. I’m focused here on the belt that drives the alternator and water pump, not the timing belt. We might as well include the belts for the air conditioning compressor and power steering pump, respectively.
Obviously, it’s a good idea to replace the drive belts regularly. As to what “regularly” means: when I started to write this article, I didn’t know a good answer to that question. I still don’t, though I read someone official-sounding suggesting that a good interval is anywhere from 40K to 70K. That seems reasonable to me. BMW probably has an official number too, but that no doubt is highly conservative, and it also presumes original BMW equipment, which most people probably don’t use on a 30-year old car.
I’ve had a drive belt fail on my 1987 BMW, and it was disconcerting to see the alternator warning light come on and then to see the temperature gauge slowly climbing while my conscience was whispering: “you DO know this engine has an aluminum head, and you know what happens to these when they overheat, yes?”
This sort situation is best avoided. I replace the timing belt on my M20-engined cars every 60K miles. Replacing the drive belts then too . . . that’s my new, improved plan.