M27 tap drill size
Drill 24.0 mm for an M27 x 3.0 tap (about 77% thread). At this diameter you would normally pilot-drill first and power-tap or thread-mill rather than tap by hand.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 23.5 mm | 23.5 | 0.9252 | 89.8% | Tight: high tapping torque, tap breakage risk |
| 15/16" (inch drill) | 23.81 | 0.9375 | 81.8% | General purpose |
| 24.0 mm (ISO 2306) | 24.0 | 0.9449 | 77.0% | General purpose |
| 24.5 mm | 24.5 | 0.9646 | 64.2% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M22 x 2.5 | 19.5 mm | 0.7677 in | 24.0 mm |
| M24 x 3.0 | 21.0 mm | 0.8268 in | 26.0 mm |
| M27 x 3.0 (this page) | 24.0 mm | 0.9449 in | 30.0 mm |
| M30 x 3.5 | 26.5 mm | 1.0433 in | 33.0 mm |
| M33 x 3.5 | 29.5 mm | 1.1614 in | 36.0 mm |
Practical notes
Drilling 24.0 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.
Thread-forming taps need about 25.5 mm (D − 0.5 × P); this is approximate, so check the tap maker's chart before drilling.
Clearance holes come from ISO 273: fine (close) 28.0 mm, medium (normal) 30.0 mm, coarse (loose) 32.0 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M27 tap?
Use a 24.0 mm drill (0.9449 in) for a cut M27 x 3.0 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread.
What clearance hole for an M27 bolt?
ISO 273 gives 28.0 mm (fine/close), 30.0 mm (medium/normal) and 32.0 mm (coarse/loose) for M27, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M27 tap?
The closest inch drill is 15/16" (0.9375 in, 23.81 mm), which gives about 82% thread on M27 x 3.0.
Is there a fine-thread M27?
Yes. A common fine pitch for M27 is M27 x 2.0 (25.0 mm drill). The coarse M27 x 3.0 (24.0 mm drill) is the default; fine pitches are chosen for adjustment, thin walls or vibration.
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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