BenchReference

M18 × 2.5 Thread

Recommended tap drill 15.5 mm
Tap drill Ø 0.6102″ / 15.5 mm
Major Ø 0.7087″ / 18 mm
Pitch 2.5 mm

The chart recommendation gives approximately 77% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

DesignationM18 × 2.5
Thread familyISO metric coarse
Nominal sizeM18
Basic major diameter 0.7087″ / 18 mm
Pitch2.5 mm
Recommended tap drill 15.5 mm (0.6102″ / 15.5 mm)
P = 2.5 mm 60° major Ø 18 mm 60° thread form (UN / ISO) — schematic

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for M18 × 2.5; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

39/64″ 77.7% 15.5 mm ★ 77% 16 mm 61.6% shaded = share of the full thread height the tap forms · accent line = drilled hole wall
★ = chart recommendation. A smaller drill leaves more of the thread height for the tap to form.
Drill Decimal in mm Approx. engagement
39/64″ 0.6094 15.478 77.7%
15.5 mm — chart recommendation 0.6102 15.5 77%
5/8″ 0.6250 15.875 65.4%
16 mm 0.6299 16 61.6%

Nearest drills to the tap drill in other systems

Drill Size vs 15.5 mm Resulting engagement
39/64″ 0.6094″ / 15.478 mm −0.022 mm 77.7%

Clearance holes

For a bolt or screw of this size to pass through freely. ISO 273 series (fine/medium/coarse), shown as close, normal, and loose.

Tapped hole Clearance hole 15.5 mm tap drill threads cut into the material 20 mm (normal fit) the tinted gap lets the bolt pass freely
Schematic — proportions exaggerated for clarity; use the table values.
Fit Drill / Ø Decimal in mm
close 19 mm 0.7480 19
normal 20 mm 0.7874 20
loose 21 mm 0.8268 21

Hex bolt & nut dimensions

For a standard hex bolt (ISO 4014/4017) and style-1 nut (ISO 4032) in this size.

Wrench / socket size (bolt head) 27 mm
Head height 11.5 mm
Nut thickness 15.8 mm
Compatible nut M18 × 2.5 hex nut
Head — top view Bolt — side view Nut — side view width across flats = 27 mm wrench head height 11.5 mm thickness 15.8 mm

Basic (nominal) dimensions. See the wrench & socket size charts for all sizes and the metric ↔ SAE socket comparison.

Related threads

Data provenance. ISO 261 / ISO 724, ISO general purpose metric screw threads (ISO). ISO 2306, Drills for use prior to tapping screw threads (ISO). ISO 273, Clearance holes for bolts and screws (ISO). ISO 4014 / ISO 4017, Hexagon head bolts and screws (ISO). ISO 4032, Hexagon regular nuts, style 1 (ISO). Engagement percentages use the conventional approximation (engagement = (major − drill) ÷ (1.29904 × pitch)) and are guidance, not a tolerance specification. See all data sources.