Does anyone know how much deflection should be on B&M 420's. The power is 509's. Thanks

Which type belt are you running?

Don't you adjust the tension on the BM blowers until the correct gap is acheived between the idler pulley bracket and mount? At least my 250 was that way.

if their from a 6,800 sc they have a spring loaded tensioner and their self adjusting..

if its an 8mm drive, about 1 inch side to side slop when fully cold will be fine.