Unified National Coarse · 8 TPI
1-8 UNC thread
Every dimension of a 1-8 UNC thread as the configurator applies it: major and minor diameter, pitch, the tap drill and the hole-size window by depth and class of fit, the thread-relief undercut, and the standard drills either side.
1-8 UNC is a Unified National Coarse thread: 1.0000 in nominal major diameter and 8 threads per inch, a pitch of 0.1250 in from one crest to the next. It is a 60° V-form thread.
The major diameter, 1.0000 in, is the outside of the external thread and the diameter a shaft end is turned to before the thread is cut. The minor diameter, 0.8647 in, is the root: the thread is 0.0676 in deep per side between the two.
The chart tap drill is 7/8 (0.8750 in), which leaves about 77 percent of a full thread. The configurator does not stop at the chart value for a tapped hole: it sizes the drill from the depth of the hole and the class of fit using the window in the second table, so it selects 7/8 (0.8750 in, 77% thread) for a hole up to a third of a diameter deep or between a third and two-thirds of a diameter deep; 22.5mm (0.8858 in, 70% thread) for a hole between two-thirds and one and a half diameters deep; 57/64 (0.8906 in, 67% thread) for a hole deeper than one and a half diameters. The drawing callout names the drill and the engagement it is based on.
1-8 UNC thread dimensions
| Dimension | Value | Unit |
|---|---|---|
| Major diameter (nominal) | 1.0000 | in |
| Minor diameter | 0.8647 | in |
| Thread depth, major to minor, per side | 0.0676 | in |
| Pitch | 0.1250 | in |
| Threads per inch (TPI) | 8 | TPI |
| Chart tap drill | 7/8 | drill size |
| Chart tap drill diameter | 0.8750 | in |
| Percent of thread at the chart tap drill | 77 | % |
| Thread-relief undercut width (3 × pitch) | 0.3750 | in |
| Thread-relief undercut diameter | 0.8497 | in |
Inches at the resolution the configurator holds them. 1-8 UNC is a 60° V-form thread. The undercut is the thread-relief groove cut ahead of a shoulder for a single-point-cut external thread, so the tool has somewhere to stop: 3 × pitch wide (DIN 76 Form A) with its root 0.0150 in below the minor diameter, which is how the configurator derives it when a thread ends in an undercut.
1-8 UNC tapped-hole limits by depth and class of fit
| Thread depth (D = 1.0000 in) | 1B/2B min Ø (in) | 1B/2B max Ø (in) | Drill the configurator selects (1B/2B) | 3B min Ø (in) | 3B max Ø (in) | Drill the configurator selects (3B) |
|---|---|---|---|---|---|---|
| To 1/3 D | 0.8650 | 0.8780 | 7/8 (0.8750 in, 77%) | 0.8650 | 0.8722 | 22.0mm (0.8661 in, 82%) |
| Over 1/3 D to 2/3 D | 0.8710 | 0.8840 | 7/8 (0.8750 in, 77%) | 0.8684 | 0.8759 | 7/8 (0.8750 in, 77%) |
| Over 2/3 D to 1.5 D | 0.8780 | 0.8900 | 22.5mm (0.8858 in, 70%) | 0.8722 | 0.8797 | 7/8 (0.8750 in, 77%) |
| Over 1.5 D (to 3 D) | 0.8840 | 0.8960 | 57/64 (0.8906 in, 67%) | 0.8760 | 0.8835 | 7/8 (0.8750 in, 77%) — nearest; no standard drill lands in this window |
Minor-diameter limits of the tapped hole in inches, from the Machinery's Handbook hole-size window. D is the 1.0000 in major diameter and the depth is the length of thread engagement; the configurator assigns the band from the hole's depth (at or under 1/3 D, 2/3 D and 1.5 D, then everything deeper) and applies the last band to any hole deeper than 1.5 D. A deeper hole gets a larger minor-diameter window, so less thread engagement, and the drill shown is the largest standard drill inside that window, with the percent of thread it leaves. Where no standard drill lands in a window the nearest is shown and marked.
Standard drills around the 1-8 UNC tap drill
| Drill | Family | Ø (in) | Percent of thread | In the 1B/2B window at 2/3 to 1.5 D |
|---|---|---|---|---|
| 27/32 (under) | fractional | 0.8438 | 96 | No |
| 55/64 (under) | fractional | 0.8594 | 87 | No |
| 7/8 (chart tap drill) | fractional | 0.8750 | 77 | No |
| 57/64 (over) | fractional | 0.8906 | 67 | No |
| 29/32 (over) | fractional | 0.9063 | 58 | No |
The two number, letter or fractional drills just under and just over the chart tap drill, with the chart drill between them. Percent of thread is (major diameter − drill diameter) ÷ (1.299 × pitch), the figure the configurator reports on the callout; a smaller drill leaves more thread and loads the tap harder. When no inch drill lands in the window the configurator searches the metric index as well, which is how a metric drill can be the pick for an inch thread in the table above.
Part of Thread and tap-drill reference. Other pages in the series: 1/4-20 UNC · 5/16-18 UNC · 3/8-16 UNC · 7/16-14 UNC · 1/2-13 UNC · 9/16-12 UNC · 5/8-11 UNC · 3/4-10 UNC · 7/8-9 UNC · 1-1/8-7 UNC · 1-1/4-7 UNC · 1-3/8-6 UNC · 1-1/2-6 UNC · 1/4-28 UNF · 5/16-24 UNF · 3/8-24 UNF · 7/16-20 UNF · 1/2-20 UNF · 9/16-18 UNF · 5/8-18 UNF · 3/4-16 UNF · 7/8-14 UNF · 1-12 UNF · M6×1 · M8×1.25 · M10×1.5 · M12×1.75 · M14×2 · M16×2 · M18×2.5 · M20×2.5 · M22×2.5 · M24×3.
Class of fit and depth of engagement for 1-8 UNC
Class of fit sets how tightly the mating threads engage: 1B and 2B share one hole window, 3B has a tighter one. Most work is 2B, and the configurator applies the 1B/2B window unless you choose 3B on the hole. The window then moves with depth: a deeper hole gets a larger minor diameter, so less thread engagement, because thread strength stops improving above roughly three-quarters engagement while every extra bit of material loads the tap.
D in the table is the 1.0000 in major diameter and the depth is the length of thread engagement. The configurator assigns a hole to a band by depth divided by D, at or under one-third, two-thirds and one and a half, and treats anything deeper as the last band; a through hole takes the band of the material it passes through. When the depth is not yet known it uses the two-thirds to one-and-a-half band.
Thread relief undercut for 1-8 UNC
A 1-8 UNC external thread that runs up to a shoulder ends in a relief groove 0.3750 in wide, three times the pitch, with its root at 0.8497 in, 0.0150 in below the minor diameter. Without it a full-depth single-point thread has nowhere for the tool to stop. Choose an undercut termination on the thread and the configurator derives that groove and prices it as the extra grooving pass it is; choose a runout and the thread simply fades with no groove.
Frequently Asked Questions
These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard; check them against your part and its mating components before ordering. See all guides on the guides page.