ISO metric · fine

M42 × 3 tap drill size

For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.

Cut tap

39.5 mm

1.5551" · 64% thread

Or 1-35/64 (1.5469") for 70%

Form tap

40.5 mm

1.5945" · 74% thread

Open M42 × 3 in the calculator

Pick your material for its % of thread, tapping RPM and the rigid-tap code.

Thread dimensions

Major Ø: 42.000 mm (1.6535")
Pitch: 3 mm
Pitch Ø (basic): 40.051 mm (1.5768")
Minor Ø (internal, basic): 38.752 mm (1.5257")
Thread depth (internal): 1.624 mm (0.0639")

Basic sizes of the 60° form. Tolerance classes (6H) set the limits around them.

Cut tap drills

For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Cut tap drills for M42 × 3
Drill	Ø in	Ø mm	Thread
1-17/32	1.5313	38.89	80%
39 mm	1.5354	39.00	77%
1-35/64suggested	1.5469	39.29	70%
39.5 mm	1.5551	39.50	64%
1-9/16	1.5625	39.69	59%
40 mm	1.5748	40.00	51%

Form tap drills

A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Form tap drills for M42 × 3
Drill	Ø in	Ø mm	Thread
1-19/32	1.5938	40.48	74%
40.5 mm	1.5945	40.50	74%
1-39/64	1.6094	40.88	55%

3-wire measurement

Best wire size: 1.732 mm (0.0682")
Over wires, at basic pitch Ø: 42.650 mm (1.6791")

For the external thread (the screw). The best wire touches the flanks at the pitch diameter.

Sources

ISO 261 / ISO 262 — ISO general purpose metric screw threads: general plan and selected sizes
OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size