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Description
Technical data for the UNF 7/8-14 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNF 7/8-14 | |
| Thread Pitch P in Inches and mm | 14 threads per inch | 1.81 mm |
| Thread Diameter d1 mm / Decimal | 22.22 mm / 0.8750 in | |
| Thread angle G | 60° | |
| Thread Standard | UNF ANSI B-1.1 | |
| Overall Length l1 | 80 mm | |
| Flute Length l2 | 22 mm | |
| Shank Diameter d2 | 18 mm | |
| Square Drive Width Across Flats a | 9 mm | |
| Tap Drill Hole | 20.4 mm | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNF 7/8-14 thread is cut into the wall of a hole with a diameter of 20.4 mm. The thread groove extends 0.91 mm into the wall. A single tap would have to cut the entire groove at once, requiring high manual torque and risking breakage. The set divides this work into two passes.
The drawing shows the entire set in operation. Both taps have the same shank and the same length. They differ in their chamfer, the tapered tip: No. 1 has a chamfer spanning 8 threads, while No. 2 spans only 2 threads. The detail windows at the hole wall show how the thread groove deepens after each tap.
| Tap | Chamfer | Chamfer angle | Material removed | How it works in the hole |
|---|---|---|---|---|
| No. 1 – roughing tap | long, 8 threads | 5° | the main portion of the groove | enters smoothly and establishes thread alignment |
| No. 2 – finishing tap | short, 2 threads | 20° | the remainder, to the full profile | screws into the cut threads and produces the full 2B profile |
How to distinguish the taps in the set
The chamfer on No. 1 is 4 times as long as the chamfer on No. 2 — the difference is immediately visible when you place them side by side. No. 1 has an incomplete thread profile; only No. 2 has the full profile. The number is also indicated on the shank: No. 1 has one ring, No. 2 has none. Keep the set together and always start with the roughing tap.
How to tap a UNF 7/8-14 hole – sequence 1, 2
Hand tapping step by step
Thread UNF 7/8-14 in four steps
ØDrilling
Drill bit 20.4 mm
1Roughing tap
Chamfer 8 threads
2Finishing tap
Chamfer 2 threads
✓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. Inch thread pitch is specified in threads per inch, so first convert it to millimeters: an inch is 25.4 mm, and 25.4 : 14 threads ≈ 1.81 mm. The UNF 7/8-14 thread diameter is 7/8 in, or 22.22 mm. The diameter of 22.22 mm minus the pitch of 1.81 mm gives 20.41 mm; in practice, use a 20.4 mm drill bit. An undersized hole is a common cause of taps twisting off in the workpiece.
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 55.6 mm, or 2.5 times the thread diameter (2.5 x 22.22 mm, shown as 2.5D in the drawing). Drill the hole deeper than the thread. The finishing tap has a chamfer spanning 2 threads, so approx. 2 incomplete threads remain near the bottom. The extra depth below the thread accommodates them along with the chips.
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 – the finishing tap produces the full profile
Unscrew the roughing tap and remove the chips from the hole. Screw the finishing tap into the cut threads by hand; it should engage them on its own. Continue turning with the same rhythm as with the roughing tap. The finishing tap removes the remaining material and cuts the groove to the full profile within tolerance 2B. When it screws in smoothly all the way, the thread is complete; check it with a UNF 7/8-14 screw.
UNF 7/8-14 hand tap – set of 2 taps for large fine threads
UNF 7/8-14 hand tap is a two-piece tool set for producing large-diameter internal Unified National Fine – UNF threads (unified fine threads). The set contains HSS roughing and finishing taps and complies with DIN 2181.
Specifications of the UNF 7/8-14 fine-thread tap
- Class 2B tolerance – ensures leak-tightness and a precise fit in hydraulic connections
- Large diameter of 7/8 inch (22.23 mm) – designed for heavy-duty connections requiring leak-tightness
- 14 threads per inch (1.81 mm pitch) – fine thread profile resists loosening under vibration
- Two-stage tapping – roughing and finishing taps ensure optimal distribution of work
- HSS high-speed steel – durability when machining materials up to 800 MPa
The UNF 7/8-14 thread is used in hydraulic power systems, pneumatic systems, and heavy construction equipment requiring leak-tight connections. The UNF 7/8-14 inch tap is chosen by workshops servicing Caterpillar and Komatsu machines and industrial hydraulic systems.
Frequently asked questions
Calculate the hole size using the formula: d = D - P. For UNF 7/8-14: d = 22.23 mm - 1.81 mm = 20.42 mm. Drill the hole with a 20.5 mm drill bit before tapping.
Both threads have a diameter of 7/8 inch (22.23 mm), but differ in pitch. UNF 7/8-14 has 14 threads per inch, while UNC 7/8-9 has only 9 threads per inch. The UNF version is finer and provides better sealing in hydraulic connections.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the outside diameter; it should be 22.23 mm (7/8 in). Internal thread (in a hole): measure the hole diameter; it should be approximately 20.5 mm. Then place the thread pitch gauge against the threads; the 14 TPI leaf fits this thread. The 60° thread angle confirms the American standard.
Product accessories
Metal drill bit 20.5 NWKc HSS DIN 345 Baildon
Reference: 13213 In stock > 10 items
€19.74
€24.28 brutto
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