M 38.5 , 1.5  =  38.5 - 1.5  =  M 37 tap drill

The above examples are for 75% thread.

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For other percentages, change the ( .75 ) in the equations below.

( inch ) Major Dia. less ( 1.299  x  .75 )  =  tap drill 
            t.p.i.

(metric) Major Dia. less ( 1.082  x  pitch  x .75 )  =  tap drill 

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FIND COUNTER BORE DEPTHS

FOR

SOCKET HEAD CAP SCREWS

example: 1/4 in. screw c'bore depth = .260

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FIND THE DISTANCE ACROSS THE CORNERS

OF A HEX SHAPE

add 15.5% to the distance across the flats

example: 5/8 hex plus 15.5% = .722

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ACROSS THE CORNERS OF A SQUARE

add 41.5% to the distance across the flats

example: 1" square plus 41.5% = 1.415