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
- 5.7608 mm
- Approx. external minor Ø
- 5.2371 mm
- Triangle height H
- 0.7856 mm
01Reading the 1/4-28 designation
The nominal major diameter is 6.35 mm; pitch is 25.4 divided by 28, approximately 0.907 mm. If the drawing adds 2A or 2B, these indicate an external or internal tolerance class, not a drill size.
02Preparing an internal thread
Allied Machine specifies a #3 drill, 0.213 inches, for cut tapping. This is exactly 5.4102 mm. Select a forming-tap hole from the tool manufacturer’s instructions rather than reusing this cutting-tap diameter.
03Avoiding substitution errors
1/4-20 shares the diameter but has a different pitch. M6 × 1 is not interchangeable either. Confirm both dimensions before repairing a thread or ordering a tool, then inspect the finished fit with the appropriate class of gauge.
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 |
|---|---|---|
| 1/4-28 UNF | 6.35 | 28 TPI · 0.9071 mm |
| 1/4-20 UNC | 6.35 | 20 TPI · 1.27 mm |
| M6 × 1 | 6 | 1 mm |
Frequently asked questions · 1/4-28 UNF
What is 1/4-28 in millimetres?
Nominal diameter is 6.35 mm. The 28 TPI correspond to approximately 0.907 mm pitch.
How do 1/4-20 and 1/4-28 differ?
Both have the same nominal diameter. The former is 20 TPI UNC; the latter is 28 TPI UNF.
Is it the same as 1/4 NPT?
No. NPT is a pipe-thread family with a different geometry and size designation.
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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