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Tap size standards
Tap size standards are mainly based on international (ISO), German (DIN), Chinese (GB), and American (ANSI) standards. Metric (M) taps commonly use major diameter × pitch (e.g., M10×1.5), while imperial (UNC/UNF) taps are specified by the number of threads per inch. Key parameters include the thread’s major diameter, pitch diameter, minor diameter, and tolerance zone (e.g., H2/D3 indicates pitch diameter accuracy). The drilling diameter is generally: tap diameter – pitch.
I. Common Tap Standard Size Types
Metric Taps: Marked as M + nominal diameter + pitch. For example, M6×1, M8×1.25, M10×1.5, M12×1.75, etc.
Imperial/American Standard Taps (UNC/UNF): Marked as thread major diameter – number of threads per inch. For example, 1/4″-20 (UNC), 10-32 (UNF).
Pipe Taps:
G (BSPP): 55-degree non-sealing pipe thread (Imperial).
NPT/NPTF: 60-degree sealing tapered pipe thread (American).
II. Key Dimensional Parameters
Major Diameter: The tap’s tip diameter, determining the minor diameter of the internal threaded hole.
Pitch Diameter: The core parameter determining the thread fit accuracy; standards are divided into H-class (metric/American) and D-class (metric).
Tolerance Zones:
H1, H2, H3, H4: Corresponding to American (ANSI) standards; H2/H3 are universal.
D1, D2, D3: Corresponding to metric (ISO/DIN) standards.
Shan Size: Includes the square dovetail size; requires a tapping machine or tap wrench.
III. Quick Reference for Common Tap Drilling (Pre-machined Hole) Diameters
M3×0.5: 2.5 M4×0.7: 3.3 mm
M5×0.8: 4.2 mm
M6×1.0: 5.0 mm
M8×1.25: 6.8 mm
M10×1.5: 8.5 mm
M12×1.75: 10.2 mm
IV. Standards Applicable
DIN Standards: DIN352 (hand taps), DIN371 (through-hole reinforced shank), DIN376 (through-hole shank), DIN374 (fine thread).
ISO Standards: International Organization for Standardization (ISO) standards.
GB Standards: Chinese National Standards.
Note: Roll taps typically have a larger drilling diameter than cutting taps because they produce threads through plastic deformation and do not generate chips.
