M18 tap drill size
The tap drill for an M18 × 2.5 metric coarse thread is 15.5 mm (0.6102 in). It leaves about 77 % thread, the normal range for steel.
Choose a drill
Each option redraws the section above with the thread it would leave.
M18 thread data
How much thread you need
Thread percentage to aim for by material.
Bars run from 50 % to 90 % thread. Typical shop practice for cut taps. Harder, tougher materials take less thread to save the tap; soft materials need more engagement to hold.
How the percentage is worked out
- D
- 18 mm, major diameter
- drill
- 15.5 mm
- P
- 2.5 mm pitch
Why not 100 %?
Going from 75 % to 100 % thread adds only about 5 % strength but roughly triples the torque needed to turn the tap. That is why the standard drill leaves 70–80 %.
For cut taps the quick rule is drill = major diameter − pitch. Form (roll) taps displace metal instead of cutting it and need a larger hole, roughly D − 0.5 P; check the tap maker's chart.
Other metric coarse threads
Common questions
What size drill for an M18 × 2.5 tap?
Use a 15.5 mm drill. It leaves about 77 % thread engagement.
What is 15.5 mm in inches?
15.5 mm is 0.6102 inch.
What size clearance hole for M18?
ISO 273 gives 19 mm for a close fit, 20 mm normal and 21 mm loose.
How tight should an M18 bolt be?
About 312 N·m for class 8.8 with dry threads.