The tap drill for a 1-8 UNC thread is a 7/8" drill (22.22 mm), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 0.8782 in, and 7/8" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.9188 in | 59/64" | 0.9219 in / 23.42 mm | 48% |
| 60% | 0.9026 in | 29/32" | 0.9063 in / 23.02 mm | 58% |
| 65% | 0.8945 in | 57/64" | 0.8906 in / 22.62 mm | 67% |
| 70% | 0.8863 in | 57/64" | 0.8906 in / 22.62 mm | 67% |
| 75% | 0.8782 in | 7/8" | 0.8750 in / 22.22 mm | 77% |
The drills either side of the target are 10 points of thread apart, so either is a reasonable choice. 7/8" (0.8750 in) gives 77% and a stronger but harder-to-tap thread; 57/64" (0.8906 in) gives 67% and less tapping torque. On 1-8 UNC the choice is mostly about tapping torque and machine power.
1-8 is used on structural bolting, foundation anchors, large presses and heavy machinery. Its basic pitch diameter is 0.9188 in and its tensile stress area is 0.6057 in². A bolt of this size passes through a 1.0940 in normal-fit clearance hole (ASME B18.2.8).
Example: 1-8 UNC at 75% thread
Result: 7/8" (0.8750 in), 77% thread
| Major (nominal) diameter | 1.0000 in / 25.40 mm |
|---|---|
| Pitch | 8 TPI (0.1250 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.9188 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.8647 in |
| Tensile stress area | 0.6057 in² |
| Fit | Hole diameter |
|---|---|
| Close | 1.0310 in / 26.19 mm |
| Normal | 1.0940 in / 27.79 mm |
| Loose | 1.1560 in / 29.36 mm |
All UNC sizes:
Use 7/8" (0.8750 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 0.8782 in. For an easier-to-tap 50% thread, 59/64" (0.9219 in) gives 48%.
7/8" is 22.22 mm. The nearest metric drill to the 75% target is 22.5 mm (0.8858 in), which gives 70% thread.
A forming tap needs a bigger hole: 0.9447 in for 65% thread, so drill 15/16" (0.9375 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 1-8 UNC, ASME B18.2.8 gives close fit 1.0310 in, normal fit 1.0940 in and loose fit 1.1560 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.6057 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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