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Description
Technical data for the UNC No 8-32 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNC No 8-32 | |
| Thread Pitch P in Inches and mm | 32 threads per inch | 0.79 mm |
| Thread Diameter d1 mm / Decimal | 4.17 mm / 0.1640 in | |
| Thread angle G | 60° | |
| Thread Standard | UNC ANSI B-1.1 | |
| Overall Length l1 | 53 mm | |
| Flute Length l2 | 13 mm | |
| Shank Diameter d2 | 4.5 mm | |
| Square Drive Width Across Flats a | 3.55 mm | |
| Tap Drill Hole | 3.5 mm | |
Roughing, intermediate, and finishing taps – how much material each removes
Division of work in the 1–2–3 set
The UNC No 8-32 thread is formed in the wall of a 3.5 mm diameter hole. The thread groove extends 0.335 mm into the wall. A single tap would have to cut the entire groove at once – by hand, this creates high resistance and a risk of breakage. The set divides this work into three layers.
The drawing shows the entire set in operation. All three taps have the same shank and the same length. They differ in chamfer length: No. 1 has a taper spanning 7–10 threads, while No. 3 spans only 1–2 threads. The detail views at the hole wall show how the thread groove deepens after each tap.
| Tap | Chamfer | Material removed | How it works in the hole |
|---|---|---|---|
| No. 1 – roughing tap | long, 7–10 threads | approx. 60% | enters smoothly and establishes thread alignment |
| No. 2 – intermediate | medium, 3–5 threads | approx. 30% | screws into the previously cut threads and starts cutting only deeper in the hole |
| No. 3 – finishing tap | short, 1–2 threads | approx. 10% + sizing | turns easily and produces the full 2B profile |
How to identify the tap numbers in the set
The chamfer is the tapered end of the tap. No. 1 has the longest chamfer and No. 3 the shortest – the difference is immediately visible when they are placed side by side. No. 1 and No. 2 have shallower thread profiles; only No. 3 has the full profile. The number can also be identified on the shank: No. 1 has one ring, No. 2 has two, and No. 3 has none. Keep the set together and always start with the roughing tap.
How to tap a UNC No 8-32 hole – sequence 1, 2, 3
Hand tapping step by step
Thread UNC No 8-32 in five steps
ØDrilling
Drill bit 3.5 mm
1Roughing tap
Removes approx. 60%
2Intermediate
Removes approx. 30%
3Finishing tap
Removes approx. 10%
✓Finished thread
Tolerance 2B
Turning sequence: half a turn clockwise, a quarter turn counterclockwise | Between taps: remove chips from the hole
Start with the hole. For inch threads, pitch is specified in threads per inch, so first convert it to millimeters: an inch is 25.4 mm, and 25.4 ÷ 32 threads ≈ 0.79 mm. The UNC No 8-32 thread diameter is 0.164 inches, or 4.17 mm. Subtracting the 0.79 mm pitch from the 4.17 mm diameter gives 3.38 mm. The table specifies a 3.5 mm drill bit: tolerance 2B allows a hole from 3.3 to 3.53 mm, and this drill bit falls within that range. An undersized hole is a common cause of taps twisting off in the material.
Through hole or blind hole – how deep to drill
The set taps both types of holes. In a through hole, the thread extends through the full material thickness. In a blind hole, cut the thread to 6.3 mm, or 1.5 times the thread diameter (1.5 x 4.17 mm, shown as 1.5D in the drawing). Drill the hole deeper than the thread. The clearance below the thread accommodates chips and the finishing tap chamfer.
No. 1 – the roughing tap establishes thread alignment
Fit the roughing tap in the tap wrench and position it perpendicular to the surface. Check the angle from two sides, for example with a square. Turn half a turn clockwise, then a quarter turn counterclockwise. Reversing breaks the chip into short pieces – a slight crunch is normal. The first threads determine whether the thread will be straight.
No. 2 and No. 3 – deepening and full profile
Unscrew the roughing tap and remove the chips from the hole. Screw the intermediate tap into the cut threads by hand – it should engage them on its own. It turns easily at first and only starts cutting deeper in the hole. The finishing tap enters with almost no resistance and cuts the final portion of the profile only at the end. When it screws in smoothly all the way, the thread is complete – check it with a UNC No 8-32 screw.
UNC No 8-32 hand tap – set of 3 pcs for common numbered thread sizes
UNC No 8-32 hand tap is a three-piece set of tools for cutting internal threads to the Unified National Coarse – UNC standard (American unified coarse thread). The set includes HSS roughing, intermediate, and finishing taps and complies with DIN 352.
UNC No 8-32 tap specifications
- Class 2B tolerance – standard accuracy for joints in electronics and mechanical applications
- Diameter No 8 (4.17 mm) – common numbered size in American devices
- 32 threads per inch (0.79 mm pitch) – UNC coarse thread profile for this size
- Three-stage tapping – roughing, intermediate, and finishing taps ensure even cutting
- HSS high-speed steel – machining steels, aluminum, and plastics up to 800 MPa
The UNC No 8-32 thread is found in computers, networking equipment, electronic enclosures, and office equipment imported from the USA. UNC No 8-32 numbered-size tap is an essential tool in IT service centers and workshops repairing American electronics.
Frequently asked questions
Calculate the hole size using the formula: d = D - P. For UNC No 8-32: d = 4.17 mm - 0.79 mm = 3.38 mm. Drill the hole with a 3.4 mm drill bit before tapping.
UNC No 8-32 is found in computer cases (fan and power supply mounting), rack cabinets, audiovisual equipment, and consumer electronics from the USA. It is one of the most common sizes in the IT industry.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the major diameter – it should be 4.17 mm (No 8). Internal thread (in a hole): measure the hole diameter – it should be approximately 3.4 mm. Then place the thread pitch gauge against the threads – the 32 TPI leaf fits this thread. The 60° thread angle confirms the American standard.
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