Hyd Roller vs Hyd Flat tappet on SM heads

And a hand drawing with no reference to the pivot location. Got it. You are wrong.

The pivot location would move depending on rocker length.

Seems like simple arc and radius geometry really.

A straight line from points a to b and then measure from that line to the arc, that’s sweep on the valve stem.
That’s how a longer rocker affects the pattern. Moving the pivot location like b3 does is dealing with variables that Bewy mentioned like getting the rocker arm centered or at a right angle at mid lift.

IMG_5379.jpeg