The interval for changing the TB on the BMW M 20 engine is 4yrs and 60 K miles. On my 1st 528e bought used with 150 k on it, I figured that at 180k, I'd get it done. The belt that came off was worn thin on the outside and starting to fray. I tore in 1/2 like it was paper. M 20 is an interference engine. That is why junkyards had plenty of 10yr old 528es and 325is in the 90s. The damage done would cost more to fix than the car was worth. Lesson learned, I used to buy a belt and tensioner for 35$ delivered. It was an afternoon's job. 
