Question 1

What drill bit do I need for an M10 tap?

Accepted Answer

Use an 8.5 mm drill bit for an M10 × 1.5 tap (standard coarse pitch). The formula is simple: tapping drill = thread diameter − pitch. So M10 − 1.5 = 8.5 mm. For M10 fine pitch (× 1.25) use 8.8 mm, and for M10 × 1.0 use 9.0 mm.

Question 2

What drill bit do I need for an M8 tap?

Accepted Answer

Use a 6.8 mm drill bit for an M8 × 1.25 tap (standard coarse pitch). M8 − 1.25 = 6.75, rounded up to 6.8 mm in practice. For M8 fine pitch (× 1.0) use 7.0 mm.

Question 3

What is the difference between a tapping drill and a clearance drill?

Accepted Answer

A tapping drill creates the pilot hole that the tap threads into — it is undersized so the tap has material to cut. A clearance drill is used when you want a bolt to pass through freely (e.g. the outer plate in a bolted joint) — it is slightly larger than the bolt's outer diameter. You need the tapping size for the component being threaded; the clearance size for the piece being clamped.

Question 4

How do I calculate tapping drill size for metric threads?

Accepted Answer

Subtract the thread pitch from the thread diameter: tapping drill = diameter − pitch. For example: M12 × 1.75 = 12 − 1.75 = 10.25 mm, which rounds to 10.2 mm in practice. This works for all standard metric threads, both coarse and fine pitch.

Question 5

What drill bit do I need for a 1/2″ BSP tap?

Accepted Answer

Use an 18.6 mm drill bit for a 1/2″ BSP tap. BSP pipe thread sizes are not calculated from diameter − pitch like metric threads — the nominal size (1/2″) does not correspond to any actual measurement on the fitting, so always use the reference table.

Question 6

What is the tapping drill size for M6?

Accepted Answer

Use a 5.0 mm drill bit for an M6 × 1.0 tap (standard coarse pitch). M6 − 1.0 = 5.0 mm exactly. The clearance drill for passing an M6 bolt through freely is 6.6 mm (medium fit).

Thread	Pitch (mm)	Tapping Drill	Clearance Drill
M2	0.40	1.60 mm	2.4 mm
M2.5	0.45	2.05 mm	2.9 mm
M3	0.50	2.50 mm	3.4 mm
M3.5	0.60	2.90 mm	3.9 mm
M4	0.70	3.30 mm	4.5 mm
M5	0.80	4.20 mm	5.5 mm
M6common	1.00	5.00 mm	6.6 mm
M7	1.00	6.00 mm	7.6 mm
M8common	1.25	6.80 mm	9.0 mm
M9	1.25	7.80 mm	10.0 mm
M10common	1.50	8.50 mm	11.0 mm
M11	1.50	9.50 mm	12.0 mm
M12common	1.75	10.20 mm	13.5 mm
M14	2.00	12.00 mm	15.5 mm
M16common	2.00	14.00 mm	17.5 mm
M18	2.50	15.50 mm	20.0 mm
M20	2.50	17.50 mm	22.0 mm
M22	2.50	19.50 mm	24.0 mm
M24	3.00	21.00 mm	26.0 mm
M27	3.00	24.00 mm	30.0 mm
M30	3.50	26.50 mm	33.0 mm
M33	3.50	29.50 mm	36.0 mm
M36	4.00	32.00 mm	39.0 mm

Tapping Drill Size Chart

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