Custom Pushrods / Preload

The relationship between the centerline of the shaft, and the valve stem tip is "geometry" on a Mopar. If you draw lines straight up thru the valve, and straight up the pedestal thru the shaft (the angle of the hold down bolts) you'll see the two lines are not parallel. They lean in towards each other. That's a big deal. That means if the shaft goes up (as with shims) or the valve tip goes up (as with poor valve jobs and/or worn parts) the distance between the two is changed. That change ruins the geometry the rockers need to have. The height of the shaft from the head gasket surface is irrelevant except for the pushrod length figure. Because you're moving both the valve and the rocker in the same direction and the same amount when a head gasket sruface in milled, the geometry is not affected.