The tap drill for a #6-32 UNC thread is a #36 drill (2.71 mm), which leaves about 78% of full thread with a cutting tap. The ideal hole for 75% thread is 0.1076 in, and #36 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.1177 in | #32 | 0.1160 in / 2.95 mm | 54% |
| 60% | 0.1136 in | #33 | 0.1130 in / 2.87 mm | 62% |
| 65% | 0.1116 in | #34 | 0.1110 in / 2.82 mm | 67% |
| 70% | 0.1096 in | 7/64" | 0.1094 in / 2.78 mm | 71% |
| 75% | 0.1076 in | #36 | 0.1065 in / 2.71 mm | 78% |
The drills either side of the target are 7 points of thread apart, so either is a reasonable choice. #36 (0.1065 in) gives 78% and a stronger but harder-to-tap thread; 7/64" (0.1094 in) gives 71% and less tapping torque. On #6-32 UNC the choice is mostly about tap breakage.
#6-32 is used for computer cases, electrical boxes, small hinges and sheet-metal brackets. Its basic pitch diameter is 0.1177 in and its tensile stress area is 0.0091 in². A bolt of this size passes through a 0.1700 in normal-fit clearance hole (ASME B18.2.8).
Example: #6-32 UNC at 75% thread
Result: #36 (0.1065 in), 78% thread
| Major (nominal) diameter | 0.1380 in / 3.51 mm |
|---|---|
| Pitch | 32 TPI (0.0313 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.1177 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.1042 in |
| Tensile stress area | 0.0091 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.1540 in / 3.91 mm |
| Normal | 0.1700 in / 4.32 mm |
| Loose | 0.1850 in / 4.70 mm |
All UNC sizes:
Use #36 (0.1065 in). It leaves 78% of full thread with a cutting tap; the ideal 75% hole is 0.1076 in. For an easier-to-tap 50% thread, #32 (0.1160 in) gives 54%.
#36 is 2.71 mm. The nearest metric drill to the 75% target is 2.75 mm (0.1083 in), which gives 73% thread.
A forming tap needs a bigger hole: 0.1242 in for 65% thread, so drill 1/8" (0.1250 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #6-32 UNC, ASME B18.2.8 gives close fit 0.1540 in, normal fit 0.1700 in and loose fit 0.1850 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.0091 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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