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
- 3.65 mm
- Approx. external minor Ø
- 3.1918 mm
- Triangle height H
- 0.6874 mm
01Identifying #8-32 UNC
The #8 screw size has a 0.164 inch nominal diameter, not 8 mm. Check 32 TPI with a pitch gauge: #8-36 UNF changes pitch, while M4 × 0.7 changes both pitch and diameter. Do not select a tap by apparent size alone.
02Tap drill in inches and millimetres
The #29 · 0.136" reference converts to 3.4544 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 #8-32 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.
Frequently asked questions · #8-32 UNC
What tap drill size is used for #8-32 UNC?
The chart lists #29 · 0.136", equivalent to 3.4544 mm. This is a cutting-tap reference; confirm its application with the tool manufacturer.
What are the diameter and pitch of #8-32 UNC?
Nominal diameter is 4.1656 mm. The 32 threads per inch correspond to approximately 0.7937 mm pitch.
Is it interchangeable with #8-36 UNF?
No. Both share the nominal diameter, but #8-32 UNC uses 32 TPI and #8-36 UNF uses 36 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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