Word of warning using 6PK1000. Although this is the part number indicated by Eurocar parts website for the number plate on my daughter's Citigo (with A/C but not stop-start function), 6PK1000 was too large!! It fitted but the belt tensioner could be turned to maximum deflection by hand all the way to the stop. The belt was very loose and you could feel the vibration with the engine running inside the car! It turned the alternator ok but squealed like pig, especially when the A/C was switched on. Purchased a 6PK989 (which funnily enough is 989mm circumference as apposed to 1000mm on the 6PK1000) from TPS and bingo, was a nice and snug fit. I used an 8mm allen key on the '6 pointed star' socket (its not a Torx socket BTW ) on the belt tensioner.
