k1 and k1 aero front belts
 

are the k1 and the k1 aero with gear diff the same bendo number belts. as my front belt seem tight and is slacken right back off.basically are the ball and gear diff the same size. mine is 396 belt

Just checked both exploded diagrams and they are the same belt. U4187

Same,have you put diff in right way round? As the pulley is off set.

Mine is tight and doesn't need the tensioner to do anything Mark, and I know I built mine right, I would guess that adding a 1 tooth longer belt would make it to slack though.