By 8mm I assume you mean the thickness and I am not sure but it has to be close to that. I bought what would be the run-of-the-mill so to speak system from Zoom for my application. It has no idler nor adjustment for spacine since I have a stock inner-primary and it is a closed system. As for the fit, when I put it together everything felt like it should with a nice fit and not to loose.

 

8mm is the distance between the round rubber teeth on the belt 8mm belt systems are the best I have several of these belt systems with 40 + thousand miles and 15 years (orig belts )