M6 Bolt Clearance: Choose a 6.4, 6.6 or 7.0 mm Hole

Use a 6.6 mm drill for a normal-clearance hole that lets an M6 bolt pass through. If you need to cut M6 × 1 threads into the hole instead, the usual drill size is 5.0 mm. TR Fastenings’ chart lists both sizes for M6.

M6 describes the thread’s nominal 6 mm diameter—not the drill bit you need for every application.

Choose the clearance fit

A clearance hole lets the bolt pass through without threading into that part. Choose its diameter to suit the assembly:

These sizes and fit descriptions come from Albany County Fasteners’ metric clearance chart. Follow the assembly drawing if it specifies a diameter or tolerance rather than choosing a fit yourself.

For example, a bracket bolted into a threaded metal plate needs two different holes: a clearance hole through the bracket and a smaller hole in the plate before tapping. Drilling both parts at 6.6 mm would not prepare the plate for a conventional M6 × 1 thread.

For the threaded part, check the pitch and whether the tap cuts or forms threads. The 5.0 mm answer is for an M6 × 1 cutting tap, not every M6 tap. See our M6 tap-drill guide for pitch-specific sizes and forming-tap requirements.

Can you use an inch bit for clearance?

These are calculated nominal diameters, using 25.4 mm per inch—not exact metric replacements:

For a simple pass-through hole, an inch bit may be acceptable if the assembly permits its actual diameter; it does not automatically meet a specified metric fit. If the drawing calls for a particular hole diameter and tolerance, use tooling and a drilling process that meet them.

If “M6” is printed on a wall anchor, threaded insert or repair kit, use that product’s hole-preparation instructions. The bolt-thread designation alone is not enough to select its installation drill.