The tap drill for an M1.6 x 0.35 thread is a 1.25 mm drill (0.0492 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 1.26 mm, and 1.25 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 1.37 mm | 1.35 mm | 1.35 mm / 0.0531 in | 55% |
| 60% | 1.33 mm | 1.35 mm | 1.35 mm / 0.0531 in | 55% |
| 65% | 1.30 mm | 1.3 mm | 1.30 mm / 0.0512 in | 66% |
| 70% | 1.28 mm | 1.3 mm | 1.30 mm / 0.0512 in | 66% |
| 75% | 1.26 mm | 1.25 mm | 1.25 mm / 0.0492 in | 77% |
The drills either side of the target are 11 points of thread apart, so either is a reasonable choice. 1.25 mm (0.0492 in) gives 77% and a stronger but harder-to-tap thread; 1.3 mm (0.0512 in) gives 66% and less tapping torque. On M1.6 x 0.35 the choice is mostly about tap breakage.
M1.6 turns up in watches, small optics, model hardware and miniature electronics, usually as a cap or pan head screw. Its basic pitch diameter is 1.37 mm and its tensile stress area is 1.3 mm². A bolt of this size passes through a 1.80 mm normal-fit clearance hole (ISO 273).
Example: M1.6 x 0.35 at 75% thread
Result: 1.25 mm (0.0492 in), 77% thread
| Major (nominal) diameter | 1.60 mm / 0.0630 in |
|---|---|
| Pitch | 0.35 mm |
| Basic pitch diameter (d − 0.6495·P) | 1.37 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 1.22 mm |
| Tensile stress area | 1.3 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 1.70 mm / 0.0669 in |
| Normal | 1.80 mm / 0.0709 in |
| Loose | 2.00 mm / 0.0787 in |
All Metric coarse sizes:
Use 1.25 mm (0.0492 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 1.26 mm. For an easier-to-tap 50% thread, 1.35 mm (0.0531 in) gives 55%.
1.25 mm is 0.0492 in. The nearest inch drill to the 75% target is #55 (0.0520 in), which gives 61% thread.
A forming tap needs a bigger hole: 1.45 mm for 65% thread, so drill 1.45 mm (0.0571 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M1.6 x 0.35, ISO 273 gives close fit 1.70 mm, normal fit 1.80 mm and loose fit 2.00 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
1.3 mm², from As = π/4 × (d − 0.9382 × P)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
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