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
- 7.021 mm
- Approx. external minor Ø
- 6.2063 mm
- Triangle height H
- 1.2221 mm
01Identifying 5/16-18 UNC
5/16-18 and M8 × 1.25 have similar diameters, but the UNC pitch is approximately 1.411 mm. Do not force a metric screw: initial engagement does not prove compatibility and may damage both threads.
02Tap drill in inches and millimetres
The F · 0.257" reference converts to 6.5278 mm. A rounded metric drill is not an exact equivalent: it changes how much material the tap removes. Select the hole preparation for the workpiece material and the tool manufacturer's instructions.
03Blind holes and finished thread inspection
For 5/16-18 UNC, allow depth for the tap chamfer and chip clearance in addition to the useful thread length on the drawing. Inspect the specified fit with the appropriate gauge. Form tapping needs its own hole size rather than this cutting-tap reference.
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 |
|---|---|---|
| 5/16-18 UNC | 7.9375 | 18 TPI · 1.4111 mm |
| 5/16-24 UNF | 7.9375 | 24 TPI · 1.0583 mm |
| M8 × 1.25 | 8 | 1.25 mm |
Frequently asked questions · 5/16-18 UNC
What tap drill size is used for 5/16-18 UNC?
The chart lists F · 0.257", equivalent to 6.5278 mm. This is a cutting-tap reference; confirm its application with the tool manufacturer.
What are the diameter and pitch of 5/16-18 UNC?
Nominal diameter is 7.9375 mm. The 18 threads per inch correspond to approximately 1.4111 mm pitch.
Is it interchangeable with 5/16-24 UNF?
No. Both share the nominal diameter, but 5/16-18 UNC uses 18 TPI and 5/16-24 UNF uses 24 TPI. Their pitches and tools differ.
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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