Theoretical thread dimensions
Single-start 60° profile. Approximate basic values without tolerance-class allowances; these are not acceptance limits or tap-drill diameters.
- Basic pitch diameter
- 8.8376 mm
- Approx. external minor Ø
- 8.2266 mm
- Triangle height H
- 0.9165 mm
01Distinguishing 3/8-16 UNC
The UNC version of this diameter uses 16 TPI. A single-start 3/8-24 thread advances 1.0583 mm per revolution. Confirm pitch before fitting a nut or chasing an existing thread.
02Q drill versus a metric substitute
The cutting-tap reference lists a Q drill, 0.332 inches: exactly 8.4328 mm. An 8.5 mm drill is not an exact conversion and produces different thread engagement. Validate a substitute against the material, tool and drawing requirements.
03Machining and inspection
Unified threads have a 60° profile. In turning, feed per revolution must match pitch; pass planning and cutting depth depend on the tool and control. An internal tap-drill diameter is not a target diameter for an external turned thread.
Similar threads: check diameter and pitch
The UNC or UNF variant with the same diameter changes pitch. A nearby metric thread also changes diameter: check both measurements before replacing a thread.
| Thread | Ø mm | Pitch |
|---|---|---|
| 3/8-24 UNF | 9.525 | 24 TPI · 1.0583 mm |
| 3/8-16 UNC | 9.525 | 16 TPI · 1.5875 mm |
| M10 × 1.5 | 10 | 1.5 mm |
Frequently asked questions · 3/8-24 UNF
Which drill is used for 3/8-24?
The consulted cut-tapping table lists Q: 0.332 inches or 8.4328 mm.
Is it interchangeable with M10?
No. 3/8-24 has a 9.525 mm nominal diameter and approximately 1.0583 mm pitch. M10 requires the specified metric pitch.
What is the feed in millimetres per revolution?
For a single-start thread, pitch is 25.4 ÷ 24 = 1.0583 mm per revolution. Confirm the control’s units and feed mode.
Sources and standard reference
Cutting-tap drill:
Allied Machine · Unified National tap drillsASME B1.1 · Unified inch threads
Nominal reference data. The drawing, tolerance class and tool instructions govern hole preparation and acceptance. Do not automatically use a cutting-tap drill size for form tapping.
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