<VV> valve seat set screw retention/tap sizes rewound again ...

[Delta Inc]

Ok, first my donation::  ( me being an expert machinist and all )
the correct drill for making a hole for tapping a ::::::::::::::
5-40 set screw is a 0.101 inches diameter .
8-32 set screw is a 0.136  inches diameter.
10-32 set screw is a 0.159 inches diameter.
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there is some range that is acceptable, and therefore you see things like a
5/32 inches diameter drill used instead of the exact recommended one ( for
the case of a 10-32), heck, it's only 0.002 or so smaller, so it works fine,
especially after your hand drill woggles it out anyway (g).
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Now, what I don't ain't be an expert on is actually how to use these to hold
valve seats in... Mr. Bob Helt mentions this is his fine book * Classic
Corvair *.   .... but how many to use ??  Bob mentions tapping the hole, but
do you then use a set screw also, or just hope the upsetting tap will do the
job ?
Me being a belt & suspenders guy ( and having dropped a valve seat in my
Turbo, at some cost in $$$ and aggravation ) .... I would use 3 per valve
seat, and do both intake and exhaust seats (g).... but anyone out there with
detailed experience ??  If I get PO and use 6 per valve seat, do I create
pinging hot spots ??
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Would it be too much to countersink the holes and part of the valve seat
with a countersink carbide cutter or grinder, and install overlapping (
flathead ) cap screws to physically hold the seats in , not just hoping the
scratches from the thread will grab the side of the valve seats ??  This is
a common pro way to hold bushings in,  would it work here, especially at
1200 degrees F or so ??
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Words would be good, video of the operation even better. (g).
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thanks everybody .. ken campbell at deltawerkes.