There's supposed to be a washer IIRC and it should come with the new tensioner. No sealant needed, at least I've never had trouble without it. Don't know about the torque, I just make it "normal tight".

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL