M8 x 1.0 tap drill size
Drill 7.0 mm for an M8 x 1.0 fine thread: the 8 mm major diameter minus the 1.0 mm pitch, giving about 77% thread.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 6.8 mm | 6.8 | 0.2677 | 92.4% | Tight: high tapping torque, tap breakage risk |
| 6.9 mm | 6.9 | 0.2717 | 84.7% | General purpose |
| 7.0 mm (ISO 2306) | 7.0 | 0.2756 | 77.0% | General purpose |
| J (inch drill) | 7.04 | 0.2770 | 74.2% | General purpose |
| 7.1 mm | 7.1 | 0.2795 | 69.3% | Hard or tough material, deep or blind holes |
| 7.2 mm | 7.2 | 0.2835 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M8 x 1.0 (this page) | 7.0 mm | 0.2756 in | 9.0 mm |
| M10 x 1.25 | 8.75 mm | 0.3445 in | 11.0 mm |
| M10 x 1.0 | 9.0 mm | 0.3543 in | 11.0 mm |
| M12 x 1.5 | 10.5 mm | 0.4134 in | 13.5 mm |
| M12 x 1.25 | 10.75 mm | 0.4232 in | 13.5 mm |
Practical notes
For through holes, a spiral-point (gun) tap pushes chips ahead and suits machine tapping; for blind holes use a spiral-flute tap and drill a few pitches deeper than the full thread you need.
Typical engagement lengths: about 1 × diameter (8 mm) in steel, 1.5 × in cast iron or brass and 2 × in aluminium. Going from about 75% to full thread adds little strength but much more tapping torque.
Thread-forming (roll) taps need a larger hole, about 7.5 mm here (D − 0.5 × P). Treat that as a starting point and check the tap maker's chart; forming suits ductile materials such as aluminium, mild steel and brass, not cast iron.
Fine pitches like M8 x 1.0 are used for adjustment screws, thin-walled parts and where resistance to loosening under vibration matters. The shallower thread makes drill size more critical: a 0.1 mm change shifts engagement by about 8 percentage points.
Clearance holes come from ISO 273: fine (close) 8.4 mm, medium (normal) 9.0 mm, coarse (loose) 10.0 mm. Use fine for location, medium as the default and coarse when hole positions vary.
FAQ
What size drill for an M8 x 1.0 tap?
Use a 7.0 mm drill (0.2756 in) for a cut M8 x 1.0 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread.
What clearance hole for an M8 bolt?
ISO 273 gives 8.4 mm (fine/close), 9.0 mm (medium/normal) and 10.0 mm (coarse/loose) for M8, whatever the pitch. Medium is the usual default.
What inch drill can I use for an M8 x 1.0 tap?
The closest inch drill is letter J (0.2770 in, 7.04 mm), which gives about 74% thread on M8 x 1.0.
How is M8 x 1.0 different from other M8 pitches?
Same 8 mm major diameter, different pitch, so a different tap drill: M8 x 1.25 (6.75 mm drill). The taps and bolts are not interchangeable with M8 x 1.0 (7.0 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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