M8 x 1 Tap Drill Size
ISO metric fine · 1 mm pitch · ISO 261 / 965-1, class 6H
What size drill bit for a M8 x 1 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 6H |
|---|---|---|---|---|
| 6.9 mm | 0.2717 | 6.90 | ✗ | |
| I | 0.2720 | 6.91 | ✗ | |
| 7 mmUse | 0.2756 | 7.00 | ✓ | |
| J | 0.2770 | 7.04 | ✓ | |
| 7.1 mm | 0.2795 | 7.10 | ✓ | |
| K | 0.2810 | 7.14 | ✓ | |
| 9/32" | 0.2813 | 7.14 | ✓ | |
| 7.2 mm | 0.2835 | 7.20 | ✗ |
By material Aluminum, brass, mild steel: 7 mm (77%) · Stainless, tool steel: 7.1 mm (69%)
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 6H minor diameter limits (6.92 mm to 7.15 mm).
M8 x 1 ISO 261, 965-1, class 6H
| Major diameter | 8 mm / 0.3150 in |
|---|---|
| Pitch | 1 mm |
| Pitch diameter (basic) | 7.351 mm |
| Minor diameter 6H | 6.917–7.153 mm |
| Thread-forming tap drill | 7.6 mm 0.2992 in |
Clearance hole ISO 273
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 8 mmthread length for full strength in steel (1 × D)
- 12 mmin cast iron or brass (1.5 × D), up to 16 mm in aluminum
- +5 mmextra drill depth below the thread for a plug tap (5 pitches), +2 mm with a bottoming tap
Shop notes
M8 x 1 is the fine pitch, and it has to be called out by name: a drawing or a bolt marked just "M8" means M8 x 1.25. Drill 7 mm here for about 77% thread.
The common mistake is drilling with the coarse thread's 6.8 mm out of habit. That drill is smaller than anything that passes the 6H minor diameter check for M8 x 1, and the result is a hole that is hard to tap and a tap that is more likely to break. Even 6.9 mm and letter I are too tight.
The inch options are better than you might expect. Letter J (0.2770 in, 74%) is the closest; letter K and 9/32 (both about 66%) still pass the minor diameter check and make sense in hard material. The 9/32 is the one most people already own.
The finer pitch also shrinks the margin. One tenth of a millimeter, from 7 mm to 7.1 mm, takes the thread from 77% to 69%, and 7.2 mm is outside the limits. On the coarse M8 the same step costs far less.