Question 1

What is the tapping drill size for M6?

Accepted Answer

The tapping drill for M6 × 1.0 (standard coarse pitch) is 5.0 mm. The formula is diameter − pitch: 6 − 1.0 = 5.0 mm. The clearance drill (for a bolt to pass through freely) is 6.6 mm. M6 is common on hydraulic valve bodies, sensor ports, and small cover plate fixings on UK farm machinery.

Question 2

What is the tapping drill size for M8?

Accepted Answer

The tapping drill for M8 × 1.25 (standard coarse pitch) is 6.8 mm. Formula: 8 − 1.25 = 6.75, rounded to 6.8 mm in practice. For M8 fine pitch (× 1.0), the tapping drill is 7.0 mm. The clearance drill for M8 is 9.0 mm. M8 is very common on pump mounting, linkage brackets, and guard fixings on agricultural machinery.

Question 3

What is the tapping drill size for M10?

Accepted Answer

The tapping drill for M10 × 1.5 (standard coarse pitch) is 8.5 mm. Formula: 10 − 1.5 = 8.5 mm exactly. For M10 fine pitch (× 1.25) the tapping drill is 8.8 mm; for M10 × 1.0 it is 9.0 mm. The clearance drill for M10 is 11.0 mm. M10 is one of the most common repair sizes on implement frames and hydraulic block mounting.

Question 4

What is the tapping drill size for M12?

Accepted Answer

The tapping drill for M12 × 1.75 (standard coarse pitch) is 10.2 mm. Formula: 12 − 1.75 = 10.25 mm, rounded to 10.2 mm. For M12 × 1.5 the drill is 10.5 mm; M12 × 1.25 is 10.8 mm; M12 × 1.0 is 11.0 mm. The clearance drill for M12 is 13.5 mm. M12 is used on three-point linkage, drawbar bolts, and gearbox mounting.

Question 5

What is the tapping drill size for M16?

Accepted Answer

The tapping drill for M16 × 2.0 (standard coarse pitch) is 14.0 mm. Formula: 16 − 2.0 = 14.0 mm. For M16 fine pitch (× 1.5) the drill is 14.5 mm. The clearance drill for M16 is 17.5 mm. M16 is used on main implement frame bolts, PTO guard mounting, and cab mounts.

Question 6

How do I calculate the tapping drill size for any metric thread?

Accepted Answer

Subtract the thread pitch from the nominal diameter: tapping drill = diameter − pitch. For M10 × 1.5: 10 − 1.5 = 8.5 mm. For M12 × 1.75: 12 − 1.75 = 10.25 mm (use 10.2 mm in practice). This formula works for all metric threads — both coarse and fine pitch. For BSP pipe threads you cannot use this formula — always use a reference table because BSP nominal sizes do not correspond to actual dimensions.

Question 7

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

Accepted Answer

A tapping drill creates a pilot hole that the tap cuts threads into — it is deliberately undersized so the tap has material to cut. A clearance drill is used when a bolt must pass through freely (for example the outer plate in a two-piece bolted joint) — it is slightly larger than the bolt's outside diameter. Use the tapping size for the component being threaded; use the clearance size for the component being clamped. This calculator shows both.

Question 8

What depth should I drill the pilot hole before tapping?

Accepted Answer

For a blind hole, drill at least 1.5× the thread diameter deeper than the required thread depth, plus a few mm for chip clearance. For example, if you need 15 mm of usable thread in an M10 hole, drill to at least 25–28 mm. A taper tap cannot cut right to the bottom of a blind hole — you need a bottoming tap for the final threads. For through-holes, drill to full depth plus 2–3 mm beyond the exit.

Question 9

What is the tapping drill size for 1/2" BSP?

Accepted Answer

The tapping drill for 1/2" BSP is 18.6 mm. BSP pipe thread sizes cannot be calculated from the nominal size — the "1/2"" designation is a nominal pipe bore measurement from old iron pipe standards, not any actual dimension on the fitting. A 1/2" BSP thread has an actual outside diameter of approximately 21 mm. Always use the reference table rather than trying to calculate from the size name.

Question 10

What cutting fluid should I use when tapping?

Accepted Answer

For mild steel: neat cutting oil or a dedicated tapping fluid like Trefolex or Tap Magic. For stainless steel: sulphurised cutting oil and slow speed with frequent chip clearing. For aluminium: WD-40 or paraffin — prevents aluminium pick-up and clogging. For cast iron: tap dry — oil forms an abrasive paste with cast iron swarf. For brass: dry or a light oil. Never tap steel dry — it is the most common cause of tap breakage.

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