Im running an X5 (non squared) the belts are very reliable. I modified mine to run the xxx4 as its thicker but also longer than the xxxs belt used. Iv ran the same belt for about erm... end of 2005 ish.
I dont have any extra belt tensioners as my belt doesnt skip too much when running a one way, only when running a front diff and thats at my local club where everyone is used to the harmonic sound of a belt skip.
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