METRICcoarse60° flank

M18 × 2.5 Thread Dimensions

M18 x 2.5 is a coarse pitch METRIC thread. Pitch is 2.5 mm, equivalent to 10.16 threads per inch. The recommended tap drill leaves roughly 75% thread engagement.

Nominal Ø18 mm
Pitch2.5 mm
Threads per inch10.16
Pitch Ø16.38 mm
Tap drill15.5 mm

Thread profile

millimetres unless stated
Thread profile — M18 x 2.5d 18d2 16.38d3 14.93mm60°P 2.5h3 1.53tap ⌀15.5
M18 × 2.5 — basic profile60° flank angle · Not to scale
Designation
M18 x 2.5
Thread system
METRIC
Series
coarse
Flank angle
60°
Major diameter
18 mm / 0.71"
Pitch
2.5 mm
Threads per inch
10.16
Pitch diameter
16.38 mm
Minor diameter (external)
14.93 mm
Minor diameter (internal)
15.29 mm
Thread depth
1.53 mm
Recommended tap drill
Ø 15.5 mm / 0.61"

How these values are derived

Basic ISO thread geometry uses the fundamental triangle height H = 0.866025 × P. External minor diameter d₃ = d − 1.226869 × P, internal minor diameter D₁ = d − 1.082532 × P. Tap drill is given as nominal diameter minus pitch, which approximates a 75% thread engagement — adjust for tough materials or deep holes.