The tap drill for a 7/8-9 UNC thread is a 49/64" drill (19.45 mm), which leaves about 76% of full thread with a cutting tap. The ideal hole for 75% thread is 0.7667 in, and 49/64" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.8028 in | 51/64" | 0.7969 in / 20.24 mm | 54% |
| 60% | 0.7884 in | 25/32" | 0.7813 in / 19.84 mm | 65% |
| 65% | 0.7812 in | 25/32" | 0.7813 in / 19.84 mm | 65% |
| 70% | 0.7740 in | 25/32" | 0.7813 in / 19.84 mm | 65% |
| 75% | 0.7667 in | 49/64" | 0.7656 in / 19.45 mm | 76% |
The drills either side of the target are 11 points of thread apart, so either is a reasonable choice. 49/64" (0.7656 in) gives 76% and a stronger but harder-to-tap thread; 25/32" (0.7813 in) gives 65% and less tapping torque. On 7/8-9 UNC the choice is mostly about tapping torque and machine power.
7/8-9 appears on large structural connections, heavy equipment and anchor bolts. Its basic pitch diameter is 0.8028 in and its tensile stress area is 0.4617 in². A bolt of this size passes through a 0.9380 in normal-fit clearance hole (ASME B18.2.8).
Example: 7/8-9 UNC at 75% thread
Result: 49/64" (0.7656 in), 76% thread
| Major (nominal) diameter | 0.8750 in / 22.22 mm |
|---|---|
| Pitch | 9 TPI (0.1111 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.8028 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.7547 in |
| Tensile stress area | 0.4617 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.9060 in / 23.01 mm |
| Normal | 0.9380 in / 23.83 mm |
| Loose | 1.0310 in / 26.19 mm |
All UNC sizes:
Use 49/64" (0.7656 in). It leaves 76% of full thread with a cutting tap; the ideal 75% hole is 0.7667 in. For an easier-to-tap 50% thread, 51/64" (0.7969 in) gives 54%.
49/64" is 19.45 mm. The nearest metric drill to the 75% target is 19.5 mm (0.7677 in), which gives 74% thread.
A forming tap needs a bigger hole: 0.8259 in for 65% thread, so drill 53/64" (0.8281 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 7/8-9 UNC, ASME B18.2.8 gives close fit 0.9060 in, normal fit 0.9380 in and loose fit 1.0310 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.4617 in², from As = 0.7854 × (D − 0.9743 ÷ n)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
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