I've never heard of an om603 failing in such a manner. Of course, I've only been hanging around MB diesels for about 10 years, so I'm a relative newbie.

I'd recommend you check the vacuum pump while it's apart. The ball bearings in the oil pan are generally associated with failure of the vacuum pump. If unlucky, the disintegrating vacuum pump jams the cam chain. Perhaps you've found another failure mode?

Conventional wisdom is to replace the vacuum pump at about 150K miles as preventative maintenance.

I wasn't aware there was a tensioner on the oil pump chain. I've occasionally heard of changing the pump chain on the old 123 diesels, but don't believe they used a tensioner.

- JimY