The pilot hole drilled before cutting an internal thread.
A geometric starting estimate for a cutting tap is hole = major diameter − pitch. Select a supported drill using a stated method or a tool-maker lookup; do not always round down. The achieved theoretical thread-height percentage depends on the chosen drill and formula convention.
Confirm the tap manufacturer’s hole range for the material, tap type and required thread. Forming taps need different hole sizes. Thread-height percentage is not axial engagement length or joint strength.
Treating major minus pitch as an exact 75% rule, or forcing a pilot outside the supported drill range to its nearest endpoint.
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