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
- 5.5251 mm
- Approx. external minor Ø
- 4.7919 mm
- Triangle height H
- 1.0999 mm
01Identifying 1/4-20 UNC
1/4-20 is a 20 TPI UNC screw thread; 1/4-28 is 28 TPI UNF. Both share a 6.35 mm nominal diameter but use different pitches. Neither M6 × 1 nor a 1/4 NPT pipe connection is an interchangeable replacement.
02Tap drill in inches and millimetres
The #7 · 0.201" reference converts to 5.1054 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 1/4-20 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 |
|---|---|---|
| 1/4-20 UNC | 6.35 | 20 TPI · 1.27 mm |
| 1/4-28 UNF | 6.35 | 28 TPI · 0.9071 mm |
| M6 × 1 | 6 | 1 mm |
Frequently asked questions · 1/4-20 UNC
What tap drill size is used for 1/4-20 UNC?
The chart lists #7 · 0.201", equivalent to 5.1054 mm. This is a cutting-tap reference; confirm its application with the tool manufacturer.
What are the diameter and pitch of 1/4-20 UNC?
Nominal diameter is 6.35 mm. The 20 threads per inch correspond to approximately 1.27 mm pitch.
Is it interchangeable with 1/4-28 UNF?
No. Both share the nominal diameter, but 1/4-20 UNC uses 20 TPI and 1/4-28 UNF uses 28 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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