Thanks for your replies. I'll go to the parts store and (probably have to order) the washer. This tensioner setup really is a PITA! I replaced the tensioner on my F250 recently, and it was a snap. One integrated assembly that cost about $50, and took a few minutes to replace. But could Mercedes make it so easy and cheap? Nooooooo! I'm going to be in for about $200 by the time I'm done, and several hours of my time. Argh!!

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine