Hi I need to replace the belt tensioner on my 1999 C200 auto (W202). The bolts holding it on are a strange type of torx looking shape. A 10mm socket fits a bit loose but a 9mm wont go on there. One is behind the mech and I tried to use a 10mm ring spanner but it's loose too. I don't want to round off the bolts but I'm unsure what size and type of spanner I should get. Can anyone tell me what these are called please?