Helpful information to everyone out there- In order to properly adjust the tension on V-belts (the style most commonly found on 123s and 126s and other models), MB (as well as Volvo, Porsche et al.) recommend by name the "krikit" belt tensioning device. It is made exclusively by the Gates Rubber Company in Denver, the same Gates that make hoses and belts. To get one, call your local Gates retailer (NAPA etc.) and order tool # "91107", cost-about $18. For the serpentine belts in today's cars that do NOT have a built-in tensioner pulley, there is the "Krikit II" made for the "micro V-belt", part # 91132 - about $25. Both measure the belt tension in Kilograms of weight/pressure applied. A certain internet German tool vendor in Tennessee sells this exact same tool as a Porsche special tool item for upwards of $80! Caveat emptor.