In service tension, as opposed to new fit tension is 90Hz. Arguably, it's more important to frequently inspect your belts and check the tension than to change them every two years and not look at them otherwise. ExactFit belts stretch less than OEM belts though, so are unlikely to become dangerously loose. As I understand it, it's the thermal expansion of the engine not being matched in the belts that is the theoretical reason for a different tension being desirable in air cooled engines. The motor expands but the belt doesn't much, so the belt becomes effectively shorter. If the rear cylinder gets hotter and expands more than the front, then the rear belt becomes effectively shorter when hot than the front one. Hence the theoretical need for different tensions.
Some time ago I found the guide below really useful and always refer to it before I do my belt changes to remind me of the process and give me a bit of confidence.... http://www.tassell.co.uk/belt_change.htm In the guide it states... The reason you have to turn the engine over (when tensioning the vertical belt) is that it then places the vertical cylinder at TDC on the firing stroke, which takes the turning force off the RH side inlet sprocket, which would otherwise place extra tension on the belt. Remember how the sprocket spun around in the picture at No.14 above? Ie when you remove the vertical belt (when horizontal is at tdc) and spring tension comes off and rotates the r/h sprocket. Hope this helps
