M18 x 1.5 tap drill size
Drill 16.5 mm for an M18 x 1.5 fine thread: the 18 mm major diameter minus the 1.5 mm pitch, giving about 77% thread.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 16.5 mm (ISO 2306) | 16.5 | 0.6496 | 77.0% | General purpose |
| 21/32" (inch drill) | 16.67 | 0.6563 | 68.3% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M14 x 1.5 | 12.5 mm | 0.4921 in | 15.5 mm |
| M16 x 1.5 | 14.5 mm | 0.5709 in | 17.5 mm |
| M18 x 1.5 (this page) | 16.5 mm | 0.6496 in | 20.0 mm |
| M20 x 1.5 | 18.5 mm | 0.7283 in | 22.0 mm |
| M22 x 1.5 | 20.5 mm | 0.8071 in | 24.0 mm |
Practical notes
Drilling 16.5 mm in one pass puts a lot of load on the spindle and workholding. Pilot-drill first, then open up to size; an undersize hole is the most common cause of a seized or broken tap at this size.
Tapping torque rises steeply with diameter. On a machine use a tapping head or rigid tapping; for one-off or high-value parts, thread milling avoids the risk of a broken tap and lets you adjust the fit. Threads in tubes or bosses this size are often single-point cut on a lathe instead.
Typical engagement lengths: about 1 × diameter in steel, 1.5 × in cast iron or brass and 2 × in aluminium. In softer materials a longer engagement does more for strength than a tighter tap drill.
Large fine-pitch threads such as M18 x 1.5 are typical on shafts, bearing locknuts and thin-walled housings, where a coarse thread would cut too deep into the wall. The shallower thread makes drill size more critical: a 0.1 mm change shifts engagement by about 5 percentage points.
Clearance holes come from ISO 273: fine (close) 19.0 mm, medium (normal) 20.0 mm, coarse (loose) 21.0 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M18 x 1.5 tap?
Use a 16.5 mm drill (0.6496 in) for a cut M18 x 1.5 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread.
What clearance hole for an M18 bolt?
ISO 273 gives 19.0 mm (fine/close), 20.0 mm (medium/normal) and 21.0 mm (coarse/loose) for M18, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M18 x 1.5 tap?
The closest inch drill is 21/32" (0.6563 in, 16.67 mm), which gives about 68% thread on M18 x 1.5. That is noticeably different from the metric drill, so use it only if the metric size is unavailable.
How is M18 x 1.5 different from other M18 pitches?
Same 18 mm major diameter, different pitch, so a different tap drill: M18 x 2.5 (15.5 mm drill). The taps and bolts are not interchangeable with M18 x 1.5 (16.5 mm drill).
Sources: ISO 261 — ISO general purpose metric screw threads, general plan (pitches) · ISO 724 — basic dimensions (minor diameter D1 = D − 1.0825 P) · ISO 2306 — drills for use prior to tapping (tap drill = D − P) · ISO 273 — clearance holes for bolts and screws · Machinery's Handbook — percentage-of-thread formula
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