The tap drill for a 7/8-14 UNF thread is 13/16" (0.8125 inch, 20.64 mm), which gives about 67% thread, the standard every tap drill chart uses.
Taps and dies for 7/8-14
Which drill gives which percentage of thread?
| Target thread | Ideal drill (in) | Nearest drill | Drill (in) | Actual thread |
|---|---|---|---|---|
| 60% | 0.8193 | 13/16" | 0.8125 | 67% |
| 65% | 0.8147 | 13/16" | 0.8125 | 67% |
| 70% | 0.8101 | 13/16" | 0.8125 | 67% |
| 75% | 0.8054 | 20.5 mm | 0.8071 | 73% |
| 80% | 0.8008 | 51/64" | 0.7969 | 84% |
What are the 7/8-14 thread dimensions?
| Series | Unified UNF |
| Pitch | 14 TPI (1.814 mm pitch) |
| Major diameter | 0.8750" / 22.22 mm |
| Basic pitch diameter | 0.8286" / 21.05 mm |
| Basic minor diameter (external) | 0.7822" / 19.87 mm |
| Standard tap drill (about 75%) | 13/16" = 0.8125" |
| Clearance, close fit (rule of thumb) | 23.0 mm = 0.9055" |
| Clearance, free fit (rule of thumb) | 61/64" = 0.9531" |
How do you tap 7/8-14 cleanly?
A thread this size needs real torque to tap by hand. A T-handle wrench is too small; use a full-size bar tap wrench, start square with a tap guide or a center in the drill press or lathe tailstock, and use cutting oil throughout. In a lathe, a thread this size is often quicker and more accurate to single-point cut than to tap.
UNF threads have a shallower thread depth than UNC at the same diameter, so the tap drill is larger and the hole leaves thinner thread flanks. That makes fine threads a poor choice in soft aluminum or brass unless engagement is long, but a good one in thin steel walls and for fine adjustment.
Other threads at this diameter: 7/8-9.
Drills and cutting fluid for tapping
Frequently asked questions
What size drill for a 7/8-14 tap?
The tap drill for a 7/8-14 UNF thread is 13/16" (0.8125 inch, 20.64 mm), which gives about 67% thread, the standard every tap drill chart uses. For hard or tough material a slightly larger drill giving about 65% thread taps much more easily; the table on this page lists it.
What size clearance hole for 7/8-14?
For a bolt to pass through, a close fit is about 23.0 mm (0.9055") and a looser free fit about 61/64" (0.9531"). These are rule-of-thumb sizes of about 3% and 8% over the 0.8750" major diameter; use the close fit where location matters and the free fit where parts must line up easily.
What is the minor diameter of 7/8-14?
The basic minor diameter of a 7/8-14 external thread is about 0.7822" (19.87 mm) and the basic pitch diameter about 0.8286" (21.05 mm). An internal thread's minor diameter is set by the tap drill, which is why the drill decides how much thread is cut.
Nearby thread sizes
Keep going
Tap drills per the standard 75% thread convention used by published charts. Clearance sizes are rules of thumb, not ASME B18.2.8 values. Not professional advice.