Hammer Boring Angle

[Farrell]

Is there ever any good reason to bore a hammer on an upright such that the
long axis of the hammer & core and the shank form an angle less than 90
degrees? I'm putting new hammers and butts on an upright and don't want to
repeat original mfg. errors. I should think the hammer/shank angle should be
exactly 90 degrees and the boring distance should be like on a grand - just
a hair longer than where the shank becomes parallel with the strings, just
to allow for a small amount of hammer wear. Original hammer core/shank angle
is about 88 degrees.

(Why is there 1/2" of felt on the top half of hammer and about 5/8" on the
bottom? Did someone file the top only, or can gravity do that much? It makes
it pretty funny looking because you can see so clearly that the flat surface
of  the string imprint at the strike point is centered well below the center
of the hammer core - a good argument for why we file the top AND the bottom
of hammers!)

Or is it better to not hold to hard and fast rules and rather put new sample
parts on piano and see if it works better with the small angle?

Close examination of the new Abel butts show that the shank hole is about a
half-shank-diameter toward the back of the piano from the original location
when the butt top is horizontal. So is it better to shorten the boring
length to make the shank parallel with the strings, or might it be better to
keep the boring length about the same and have the shank never quite get to
parallel with the strings (and thus exaggerating the present hammer/shank
angle) - but having the hammer hit the strings when it is perpendicular?

A confusing array of possibilities here. I realize on the upright you likely
have a bit of geometrical leeway anyway, but even if I don't achieve
perfection, I want to push myself in that direction as far as is reasonable.
To do that I need to understand what perfection is (theoretical at least),
or our best guestimate of perfection.

"It's always something!"
Terry Farrell