The tap drill for a #8-32 UNC thread is a #29 drill (3.45 mm), which leaves about 69% of full thread with a cutting tap. The ideal hole for 75% thread is 0.1336 in, and #29 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.1437 in | #27 | 0.1440 in / 3.66 mm | 49% |
| 60% | 0.1396 in | #28 | 0.1405 in / 3.57 mm | 58% |
| 65% | 0.1376 in | #29 | 0.1360 in / 3.45 mm | 69% |
| 70% | 0.1356 in | #29 | 0.1360 in / 3.45 mm | 69% |
| 75% | 0.1336 in | #29 | 0.1360 in / 3.45 mm | 69% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 18 points of thread apart. #30 (0.1285 in) gives 87% and a stronger but harder-to-tap thread; #29 (0.1360 in) gives 69% and less tapping torque. On #8-32 UNC the choice is mostly about tap life in hard materials.
#8-32 is common in electrical enclosures, switch plates, fixtures and light machinery. Its basic pitch diameter is 0.1437 in and its tensile stress area is 0.0140 in². A bolt of this size passes through a 0.1960 in normal-fit clearance hole (ASME B18.2.8).
Example: #8-32 UNC at 75% thread
Result: #29 (0.1360 in), 69% thread
| Major (nominal) diameter | 0.1640 in / 4.17 mm |
|---|---|
| Pitch | 32 TPI (0.0313 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.1437 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.1302 in |
| Tensile stress area | 0.0140 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.1800 in / 4.57 mm |
| Normal | 0.1960 in / 4.98 mm |
| Loose | 0.2130 in / 5.41 mm |
All UNC sizes:
Use #29 (0.1360 in). It leaves 69% of full thread with a cutting tap; the ideal 75% hole is 0.1336 in. For an easier-to-tap 50% thread, #27 (0.1440 in) gives 49%.
#29 is 3.45 mm. The nearest metric drill to the 75% target is 3.4 mm (0.1339 in), which gives 74% thread.
A forming tap needs a bigger hole: 0.1502 in for 65% thread, so drill #25 (0.1495 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #8-32 UNC, ASME B18.2.8 gives close fit 0.1800 in, normal fit 0.1960 in and loose fit 0.2130 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.0140 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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