threadsizes

M8 Tap Drill Size (M8 x 1.25)

ISO metric coarse · 1.25 mm pitch · ISO 261 / 965-1, class 6H

What size drill bit for a M8 tap?

6.8 mmmetric drill
0.2677inch
6.80mm
74%thread
No 6.8 mm drill? Use 17/64" (77%) or H (77%).

To scale section of the tapped hole

Section of a M8 x 1.25 tapped hole to scale: 6.8 mm (0.2677) drill, thread length 1.5 times the diameter. ⌀8 mm P 1.25 mm DRILL 6.8 mm Thread 1.5 × D +5 P drill
Drawn from the M8 x 1.25 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for M8 x 1.25
DrillInchmmThread6H
G 0.2610 6.63 84% ✗
6.7 mm 0.2638 6.70 80% ✓
17/64" 0.2656 6.75 77% ✓
H 0.2660 6.76 77% ✓
6.8 mmUse 0.2677 6.80 74% ✓
6.9 mm 0.2717 6.90 68% ✓
I 0.2720 6.91 67% ✓
7 mm 0.2756 7.00 62% ✗
J 0.2770 7.04 59% ✗

By material Aluminum, brass, mild steel: 6.8 mm (74%) · Stainless, tool steel: 6.9 mm (68%)

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 (6.65 mm to 6.91 mm).

M8 x 1.25 ISO 261, 965-1, class 6H

Major diameter8 mm / 0.3150 in
Pitch1.25 mm
Pitch diameter (basic)7.188 mm
Minor diameter 6H6.647–6.912 mm
Thread-forming tap drill7.4 mm 0.2913 in

Clearance hole ISO 273

Close8.4 mm8.4 mm
Normal9 mm9 mm
Loose10 mm10 mm

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
  • +6.25 mmextra drill depth below the thread for a plug tap (5 pitches), +2.5 mm with a bottoming tap

Shop notes

Plain "M8" means this thread, the 1.25 mm coarse pitch. The usual 6.8 mm drill gives about 74% thread. The diameter minus pitch rule lands between the 6.7 mm and 6.8 mm drills, and both work: 6.7 mm gives 80%, which is a lot of tap torque in steel for very little gain.

This is a size where an inch-only shop is not at a disadvantage. A 17/64 (0.2656 in, 77%) and a letter H (0.2660 in, 77%) are both closer to the textbook figure than the 6.8 mm. On the loose side, 6.9 mm (68%) and letter I (67%) still pass the 6H minor diameter check and are the ones to use in hard material.

Watch the 7 mm drill. It is the correct drill for M8 x 1, and in a coarse M8 hole it leaves 62% thread and falls outside the limits. Mixing up the two M8 pitches is the most common way to end up with that hole.

The inch thread nearest in size is 5/16-18: 0.3125 in against 0.3150 in, with an 18 tpi pitch (1.4111 mm) against 1.25 mm. They look the same until one binds in the other's hole a turn or two in.