Question 1

What drill bit do I need for an M6 tap?

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. M6 is one of the most common sizes on UK farm machinery — used on hydraulic valve bodies, sensor ports, electrical brackets and cover plates.

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 to 6.8 mm in practice. For M8 fine pitch (× 1.0) use 7.0 mm. M8 is common on pump mounting, linkage brackets, and guard fixings throughout agricultural machinery.

Question 3

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). M10 − 1.5 = 8.5 mm. For M10 fine pitch (× 1.25) use 8.8 mm; for M10 × 1.0 use 9.0 mm. M10 is one of the most common repair sizes on implement frames and hydraulic block mounting.

Question 4

What drill bit do I need for an M12 tap?

Accepted Answer

Use a 10.2 mm drill bit for an M12 × 1.75 tap (standard coarse pitch). M12 − 1.75 = 10.25, rounded to 10.2 mm. For M12 fine × 1.5 use 10.5 mm; M12 × 1.25 use 10.8 mm; M12 × 1.0 use 11.0 mm. M12 is very common on three-point linkage, drawbar fittings, and gearbox mounting.

Question 5

What drill bit do I need for an M16 tap?

Accepted Answer

Use a 14.0 mm drill bit for an M16 × 2.0 tap (standard coarse pitch). M16 − 2.0 = 14.0 mm. For M16 fine × 1.5 use 14.5 mm. M16 is common on main implement frame bolts, PTO guard mounting, and cab mounts.

Question 6

What is the tapping drill size for Whitworth threads?

Accepted Answer

Whitworth (BSW) tapping drill sizes: 1/4" BSW = 5.1 mm, 3/8" BSW = 7.9 mm, 1/2" BSW = 10.5 mm. Whitworth threads are found on pre-1970s British machinery, older David Brown, Fordson and early Massey Ferguson tractors. They are identified by a rounded thread form and the TPI does not match either metric or UNF. Always use the reference table — unlike metric, you cannot calculate Whitworth tapping drill sizes from the thread name.

Question 7

What is the difference between Whitworth (BSW) and UNF threads?

Accepted Answer

Both are imperial thread forms using inches, but they have different thread angles and pitches. Whitworth (BSW) uses a 55° thread angle and is the British standard — common on pre-1970s UK machinery and older tractors. UNF (Unified Fine) and UNC (Unified Coarse) use a 60° thread angle and are the American standard — found on US-manufactured equipment and some European machinery built for export markets. The threads are not interchangeable. Use a thread pitch gauge to identify which standard you are dealing with.

Question 8

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 cuts the thread. A clearance drill is used when a bolt must pass through freely (the outer plate in a bolted joint) — it is slightly larger than the bolt's outer diameter. Use the tapping size for the threaded component; the clearance size for the part being clamped.

Question 9

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 (use 10.2 mm in practice). This works for all metric threads — both coarse and fine. For Whitworth and BSP threads you cannot use this formula — always use the reference table.

Question 10

What lubricant should I use when tapping threads?

Accepted Answer

For mild steel: cutting or tapping oil (Trefolex, Tap Magic, or neat cutting oil). For stainless steel: sulphurised cutting oil, slow speed, and back off frequently. For aluminium: WD-40 or paraffin — prevents the tap clogging with aluminium pick-up. For cast iron: tap dry — oil forms an abrasive paste with cast iron dust. For brass: dry or light oil.

Question 11

What is the difference between a taper tap, plug tap and bottoming tap?

Accepted Answer

Taper taps (first taps) have 8–10 chamfered lead threads — always use these to start. Plug taps (second taps) have 3–5 chamfered threads and do most of the cutting. Bottoming taps have 1–2 chamfered threads and cut right to the base of a blind hole. For through-holes, a plug tap alone is usually sufficient. Always start with a taper tap in blind holes and finish with a bottoming tap if you need full thread depth at the base.

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

Frequently Asked Questions

Tap & Die Set Buying Guide — What to Look For

HSS vs Cobalt vs Carbon Steel Taps

What a Good Set Should Include

Common Mistakes When Tapping Threads

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