Metric threads

The tapping drill, and why it isn't the minor diameter

The shop rule is simple: nominal diameter minus pitch. An M8 at 1.25 pitch is drilled 6.75, rounded to 6.8 which is the stock drill. An M10 at 1.5 is drilled 8.5. An M6 at 1 is drilled 5.

This rule doesn't give the theoretical minor diameter of the thread, which would be 6.65 for an M8. The difference is deliberate. Drilling to the theoretical size would produce 100 % thread engagement, which adds almost nothing to pull-out strength but greatly increases the tapping torque. The tap binds, heats, and eventually snaps in the hole. The standard aims at about 65 to 70 % engagement, the point where strength is already at its useful maximum.

Coarse or fine pitch

At the same diameter, a fine pitch advances less per turn. Three consequences. It resists loosening better under vibration, because the helix angle is shallower. It weakens the part less, since the core stays thicker. And it clamps harder at the same torque, by screw action.

Against that, a fine pitch galls more easily, tolerates repeated assembly and disassembly poorly, and is less forgiving of a cross-threaded start. Without a specific reason, coarse remains the right choice.

The clearance hole

This is the plain hole in the part the screw passes through, with no thread. It must leave play to absorb misalignment between the two parts. The medium series of ISO 273, the one in the table above, suits the great majority of assemblies: 6.6 for an M6, 9 for an M8, 11 for an M10.

The close series, tighter, is used when positioning must be precise. The coarse series serves the opposite case, assembling parts whose hole centres are loosely controlled, typically in fabricated work.

Reading a designation

M8 on its own means an M8 at coarse pitch, so 1.25. The pitch is only written when it's fine: M8×1. A full designation may add the tolerance class, M8×1-6H for a tapped hole, 6g for a screw, on the same upper-case and lower-case principle as ISO 286 fits.

Frequently asked questions

What drill for an M8 tapped hole?

6.8 mm. The general rule is nominal diameter minus pitch: 8 − 1.25 = 6.75, rounded to the nearest stock drill.

Why not drill to the theoretical minor diameter?

Because the thread would then be 100 % engaged, which adds almost no strength but multiplies the tapping torque and breaks taps. The standard aims at roughly 65 to 70 % thread engagement.

What is the difference between coarse and fine pitch?

At the same diameter, a fine pitch advances less per turn. It resists loosening under vibration better and weakens the part less, but galls more easily and tolerates repeated assembly poorly.

What is a clearance hole?

The plain, unthreaded hole in the part the screw passes through. The ISO 273 medium series gives 9 mm for an M8 screw, leaving the play needed for alignment.