The pilot hole drilled before cutting an internal thread.
Sized for roughly 75% thread engagement in steel, using drill = major diameter − pitch rounded down to the nearest standard drill. The percentage matters: more engagement gives a stronger thread but a higher risk of breaking the tap; less engagement risks stripping the thread.
In harder materials use one drill size larger, in softer materials one size smaller. The nominal 75% figure assumes steel.
Rounding the computed pilot diameter up instead of down, which reduces engagement and weakens the thread.
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