Help, removing tensioner shock

[Guenter]

I'm glad I've discovered this great forum. Have enjoyed reading past posts
and learned considerably. At idle my '93 190E makes considerable noise which my local MB service man diagnosed as the shock for the tensioner.
Idler comes off no problem, but the apparent T30 Torx screw that holds on the shock I can not get off. I noted on the new shock that the collet is threaded and has an opening for a 'captured' nut. This nut then basically
appears to act as a lock nut which might explain why I can't get the torx screw out. In addition, this torx screw does not have a particularly deep recess..... is there some trick to removing this screw? Or should I just grind it off? Any pointers would be much appreciated.

cheers, guenter

ps this difficulty is really scarrying from doing a water pump replacement on my '89 300E. Is the water pump replacement a job for the dealer?