A failed (original) vac pump can take out the timing chain. So if you recently bought an 87 300D, you should replace the vac pump post-haste if it's not the new style. Look for four Torx screws on the front cover. If it has them, you're fine, it's the newer pump. If it doesn't have the screws, install a new pump ASAP.
The timing chain itself almost NEVER fails if the oil has been changed at a reasonable interval. You don't change it for fun, only if it's stretched more than 4 degrees, per the OM603 factory engine manual. It's not an easy or fun job - I've done it on my car, and posted the results here. Search the diesel forum if you want to read it... but don't lose sleep about a chain "failing". These aren't VW timing belts, for cryin' out loud! Who is this "pro" rebuilder anyway?
