Thread Pitch & Tap Drill Chart.
Find the right tap drill, pitch, major and minor diameter for any thread. Metric coarse and fine, UNC, UNF, BSPP, and NPT — all in one place.
Thread engagement calculator
For a tapped hole, calculate the percent thread engagement based on hole diameter and minor diameter of thread. Aim for 60–75% engagement for best balance of strength and tapping ease.
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Thread Pitch Reference
Threads are specified by diameter and pitch (or threads per inch). Mismatched thread series cross-thread under load — always verify both before assembly.
Thread Series Comparison
- ISO Metric Coarse (M): e.g., M10×1.5 — most general engineering
- ISO Metric Fine (Mf): e.g., M10×1.25 — fine adjustment, vibration resistance
- UNC (Unified Coarse): e.g., 3/8-16 — US general
- UNF (Unified Fine): e.g., 3/8-24 — automotive, aerospace
- NPT (Tapered pipe): e.g., 1/2-14 NPT — fluid sealing
- BSPP / BSPT: British Standard pipe — international fluid systems
When to Use Coarse vs Fine
- Coarse: tolerates damage, faster assembly, better for soft materials. General purpose.
- Fine: higher preload at same torque, better fatigue resistance, thinner wall sections OK. Use for engine, hydraulic, instrument joints.
Tap Drill Selection
Tap drill diameter = nominal − pitch (approximate, ~75% thread engagement)
- M6 × 1.0 → 5.0 mm tap drill
- M8 × 1.25 → 6.8 mm tap drill
- M10 × 1.5 → 8.5 mm tap drill
- M12 × 1.75 → 10.2 mm tap drill
- 1/4-20 UNC → 0.201" (#7 drill)
- 3/8-16 UNC → 0.3125" (5/16" drill)
Common Thread Mistakes
- Mixing UNC and metric — they look similar but cross-thread.
- Wrong tap drill for desired thread engagement (75% vs 65%).
- NPT requires sealant or tape; BSPP requires gasket — they're not interchangeable.
- Left-hand vs right-hand confusion — always specify on drawing.
Related Tools
For bolt strength and torque, see Bolt Torque Calculator. For drill sizes, Drill Size Chart. Document thread spec in AS9102 Form 3.