Formula: tapping drill = thread diameter − pitch. Clearance drill = medium fit (ISO 273). Highlighted rows are the most common workshop sizes.
Metric tapping drill formula
Tapping drill = Diameter − Pitch
e.g. M10 × 1.5 → 10 − 1.5 = 8.5 mm | M12 × 1.75 → 12 − 1.75 = 10.25 mm (use 10.2 mm)
Frequently Asked Questions
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 fleet and plant machinery — used on hydraulic valve bodies, sensor ports, electrical brackets and cover plates.
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 fleet and plant machinery.
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 frames and hydraulic block mounting.
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.
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 frame bolts, PTO/driveline guard mounting, and cab mounts.
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 and equipment. 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.
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.
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.
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.
For mild steel: cutting or tapping 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.
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.
Tap & Die Set Buying Guide — What to Look For
HSS vs Cobalt vs Carbon Steel Taps
Carbon steel taps are cheap and fine for soft materials like aluminium and brass, but go blunt quickly in steel. High Speed Steel (HSS) taps handle general workshop use in mild steel well — what most good-quality metric sets use. Cobalt (HSS-Co) taps cost more but run hotter without failing, making them the right choice for stainless steel, hardened steel and high-production work. For a fleet or plant workshop dealing mainly with mild steel, a quality HSS set is the practical choice.
What a Good Set Should Include
Prioritise a set covering M6, M8, M10, M12 and M16 — these are the sizes you will encounter most often on machinery, hydraulic fittings and fabrication. Each size needs at minimum a taper and plug tap. A tap wrench that fits your largest tap is essential.
Hydraulic adaptors
BSP threads
Machinery bolts
M8, M10, M12
Fabrication / welding
M6, M8, M10
Frames & attachments
M10, M12, M16
Older British tractors
Whitworth (BSW)
US/JD equipment
UNF sizes
Common Mistakes When Tapping Threads
Drilling undersized — always check the tapping drill against this chart before drilling
Not starting square — use a drill press; a tap started at an angle will snap when corrected
Forgetting to break the chip — back off half a turn for every full turn in steel to clear swarf
Using the wrong lubricant — dry on cast iron, oil on steel, WD-40 on aluminium
Using a taper tap in a blind hole — finish with a plug or bottoming tap for full depth