M8 tap drill size
Use a 6.75 mm drill for an M8 x 1.25 coarse thread. That is 8 mm minus the 1.25 mm pitch and leaves about 77% thread, the usual balance of strength and tapping torque. Most shops use a 6.8 mm drill, which gives 74% thread.
| Drill | mm | inch | Thread % | Typical use |
|---|---|---|---|---|
| 6.6 mm | 6.6 | 0.2598 | 86.2% | Tight: high tapping torque, tap breakage risk |
| 6.7 mm | 6.7 | 0.2638 | 80.1% | General purpose |
| 17/64" (inch drill) | 6.75 | 0.2656 | 77.2% | General purpose |
| 6.75 mm (ISO 2306 nominal) | 6.75 | 0.2657 | 77.0% | General purpose |
| 6.8 mm | 6.8 | 0.2677 | 73.9% | General purpose |
| 6.9 mm | 6.9 | 0.2717 | 67.7% | Hard or tough material, deep or blind holes |
| 7.0 mm | 7.0 | 0.2756 | 61.6% | Hard or tough material, deep or blind holes |
| Thread | Tap drill | Tap drill (in) | Clearance (ISO 273 medium) |
|---|---|---|---|
| M5 x 0.8 | 4.2 mm | 0.1654 in | 5.5 mm |
| M6 x 1.0 | 5.0 mm | 0.1969 in | 6.6 mm |
| M8 x 1.25 (this page) | 6.75 mm | 0.2657 in | 9.0 mm |
| M10 x 1.5 | 8.5 mm | 0.3346 in | 11.0 mm |
| M12 x 1.75 | 10.25 mm | 0.4035 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 7.38 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.
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 tap?
Use a 6.75 mm drill (0.2657 in) for a cut M8 x 1.25 tap, per ISO 2306 (major diameter minus pitch). It gives about 77% thread. A 6.8 mm drill is the common practical choice (74% 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 tap?
The closest inch drill is 17/64" (0.2656 in, 6.75 mm), which gives about 77% thread on M8 x 1.25.
Is there a fine-thread M8?
Yes. A common fine pitch for M8 is M8 x 1.0 (7.0 mm drill). The coarse M8 x 1.25 (6.75 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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