A tap-drill recommendation helps create material for an internal thread, but the finished pitch diameter also depends on the tap, machine, material, coating and process. Class therefore cannot be inferred from pre-hole size alone.
Unified classes
Unified designations pair a class number with A for external threads or B for internal threads. The drawing or governing specification controls which class applies; a familiar general-purpose class is not permission to substitute it.
Metric tolerance classes
ISO metric tolerance designations combine a grade number and fundamental-deviation letter. Internal and external threads use different conventions, and coatings may consume available allowance.
What the drill can and cannot control
The pre-hole affects thread height and tapping torque. It does not independently set pitch diameter, lead error, flank angle or surface condition. Inspect the characteristic the drawing actually requires.
Use the current licensed standard and qualified inspection plan for acceptance limits. This page explains notation only.
Technical references
- ASME B1.1-2024 Unified Inch Screw Threads — ASME. Accessed 2026-11-10.
- ISO 261 ISO general purpose metric screw threads — General plan — International Organization for Standardization. Accessed 2026-11-09.
Standards references identify scope and terminology; protected dimensional tables are not reproduced.