On Jan 13, 3:38*pm, "
wrote:

So what would be so hard about adapting a propeller duplicating
pantograph to the task of finish machining a rough casting?
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Neat play on words :-)
The basic problem is one of rigidity. The initial cut must also be
your finished cut, with regard to width. Unfortunately, air heats as
it expands. If the walls of the fin are perpendicular then it's
ability to couple heat to the air becomes a function of the width of
the channel between the perpendicular 'walls' of your fins, as well as
the pressure of the air flowing in that perpendicularly walled
canyon. As it is, with the amount of draft found in the typical
casting, you need one hell of a lot of air-pressure for the thing to
work properly. In other words, if all you can do is make a vertically
sided cut, then the EFFECTIVE depth of the thing becomes a function of
its width. Since your width is also a function of the rigidity of
your tooling, with a width of an eighth of an inch or there abouts,
your effective depth is reduced to about half an inch. To go any
deeper, the walls would have to taper, or be stepped, or whatever.
This is based on the thermal transfer equations found in Taylor,
Liston and others.
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... The tough part was
getting the old seats out.
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Joke, right?

To remove swaged-in seats from aluminum heads you fire up your buzz-
box, fill your coffee cup with ice water, scratch an arc on one side
of the seat then weave a bead about 5/8" long x 3 passes 'deep,'
transfer your arc to the OPPOSITE side of the seat and do the same...
then dash your cup of iced water on the thing. Amidst the frying and
the hissing and the steam getting under your helmet you'll hear a
musical little PING! and your valve seat will be cocked up at an
angle, easily grasped with a pair of vise-grips welded to a barrel-
nut, which you've screwed to the business-end of your slide-hammer.
Give it a couple of slaps and there's your valve seat.

-R.S.Hoover