Here is what Comp Cams says about it, although I like Bob's thinking
better.
http://compcams.com/information/Products/Pushrods/
Craig
Palmer, Robert L. wrote:
> Frank,
>
> It seems to me that changing push rod length can satisfy one or the other
> criterion, but not both at the same time; either you get maximum extension at
> half lift (the 90 degree angle point), or you center the contact patch on the
> center of the valve stem. If the rocker arm length equals the stud/valve stem
> separation, then everything is fine; otherwise, some compromise will be
> needed.
>
> Here's some instructions on how to pick the push rod length I found, which I
> believe is what you described:
>
> "It is desirable to have the least possible amount of roller movement across
> the valve tip as possible. This will provide maximum valve lift and minimum
> wear and side loading of the valves and guides.
>
> To determine the roller movement across the valve tip, known as the "rocker
> strike" or "contact patch". Apply a thin smear of bearing blue (Prussian blue)
> to the valve tip, fit rocker arm and set lash clearance. Engines fitted with
> hydraulic lifter cams set clearance to zero.
>
> Rotate the engine by hand at least on full turn watching the roller movement
> across the valve, the roller should move forward across the tip until half
> lift and them back to its starting position at full lift.
>
> The 'rocker strike' or 'contact patch' should be central on the valve unless
> non standard length valves are fitted due to the intersecting angles of the
> stud and valve centerlines.
>
> Geometry can be changed by fitting different length pushrods or valve lash
> caps, shorter pushrods or lash caps will bring the starting point further back
> from the center of the valve, longer pushrods will have the opposite effect.
>
> Ideal geometry will have a strike area of approximately .060" [1.5mm] wide,
> central on the valve and have the major amount of roller movement at low lift
> where the spring pressure is least.
>
> Before running the engine it is vital to ensure free operation and at lease
> .060" [1.5mm] of clearance between arm and valve spring, retainer, base of
> rocker stud and back of posilok nut.
> Clearance should be checked at both closed and fully open valve positions."
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