The hex bit must be a special stubby socket. If you can find one somewhere fine but the ratchet still may hit the radiator. The bolt with nuts worked fine for me and there was plenty of room. FWIW I could not find a stubby hex socket or for that matter a hex socket that size. I suppose I could have ordered one from mercedes or bought their tools for $$$. The bolt and nuts cost less than $2.00.

The tensioner does not have to come off to replace either the spring or the shock or the belt. The tensioner does however break and I would replace it. The tensioner spring tensions the belt no adjustment is needed. Put the belt on the tensioner pulley first smooth side on pulley. I used a coat hanger to pull it through. Once the belt is on the pulley the belt virtually falls into place all you have to do is put it on the pulleys. An aside I understand that someone makes a sturdier tensioner if you can find one.