Threading Taps

Measure the major diameter with calipers, then check the pitch with a thread gauge. A 60° flank angle with a round metric pitch means Metric. A 55° flank angle puts you in the Whitworth family — BSW, BSF or one of the BSP variants. 60° with a threads-per-inch count means UNC, UNF or NPT. A 47.5° flank is BA, and nothing else uses it.

If the thread narrows along its length it's a taper thread, which leaves you with BSPT or NPT. A thread pitch gauge costs a few pounds and removes the guesswork entirely.

Flank angle, mostly. BSP runs 55°, NPT runs 60°, so the two will never seat properly against each other even where the diameters look close. The pitches differ too — 1/4″ BSP is 19 TPI against 18 TPI on 1/4″ NPT.

Force one into the other and you damage the softer of the two parts and end up with a joint that weeps under pressure. BSP is the standard across UK and European hydraulics; NPT arrives with American-built plant and compressed air equipment.

Look at how the joint seals. BSPP (marked G) is parallel: the thread holds the fitting and the seal comes from a bonded washer, O-ring or captive seal on the face. BSPT (marked Rc or R) is tapered at 1:16, so it wedges as it tightens and seals on the thread flanks with PTFE tape or a liquid sealant.

Flat sealing face on the port means BSPP. Plain tapered hole in a manifold or casting means BSPT.

For metric coarse threads, subtract the pitch from the major diameter. M6 × 1.0 needs a 5.0mm drill, M8 × 1.25 needs 6.8mm, M10 × 1.5 needs 8.5mm and M12 × 1.75 needs 10.2mm.

That gives roughly 75% thread engagement, which is the practical sweet spot. Going deeper adds very little holding strength and a great deal of tap breakage. Forming (roll) taps are the exception — they need a larger hole, around 5.6mm for M6.

It comes down to the lead, meaning how many threads are ground away at the tip. A taper tap has 8 to 10 threads of lead, so it starts square easily and is the one to open a hand-tapped hole with. A second or intermediate tap has 3 to 5 and does most of the cutting. A plug or bottoming tap has 1 to 2, which lets it cut almost to the floor of a blind hole.

A three-piece hand set gives you all three and covers most workshop jobs.

Match it to the hole. Spiral point (gun nose) pushes swarf forward and out of the bottom, so it belongs in through holes and it's the fastest option on a machine. Spiral flute pulls swarf back up and out, which is what a blind hole needs because chips have nowhere else to go.

Straight flute is the general-purpose hand tapping choice and works well in short-chipping materials such as cast iron and brass.

On ductile material — aluminium, copper, mild steel and low-carbon stainless. A roll tap displaces metal instead of cutting it, so there is no swarf at all. That removes the single biggest cause of breakage in blind holes, and the thread comes out stronger because the grain flow follows the form rather than being cut through it.

The catch is the drill. A forming tap needs a larger hole than a cutting tap: around 5.6mm for M6 rather than 5.0mm. Use the cutting-tap drill and you will snap it.

HSS handles mild steel, aluminium and general workshop material, and it's the cheapest per hole. HSS-E is cobalt-alloyed, keeps its edge at higher temperature and is the sensible pick for stainless, tool steel and tougher alloys.

Solid carbide is for cast iron, abrasive materials and high-volume production on a rigid machine. It lasts far longer but it's brittle, so any misalignment or spindle deflection will break it.

Nine times out of ten the tapping drill is undersized. A 4.8mm hole where an M6 tap wants 5.0mm roughly doubles the torque on the tap.

After that, in order: swarf packing into a blind hole (switch to a spiral flute), the tap entering off-square, no cutting fluid, or a worn tap being pushed past its life. If you're rigid tapping on CNC, check the feed matches the pitch exactly — even a small mismatch loads the tap sideways and snaps it.

No. An M6 tap and a 1/4″ BSW tap sit within about half a millimetre of each other on diameter and look near-identical in the drawer, but the flank angles are 60° and 55° and the pitches are different.

Run the wrong one through and you either strip the hole or end up with a fastener that binds partway down and never seats. Identify the thread form before you pick the tap.

British Association — a miniature metric-based standard with a 47.5° flank angle, which nothing else uses. 0BA is 6.0mm major diameter with a 1.0mm pitch, and the sizes get smaller as the number rises, down to 25BA.

It's still specified for instrument work, model engineering, older electrical switchgear and heritage restoration, so it tends to be a thread you either never touch or need constantly.

Yes. Left-hand metric taps for anti-rotation applications, metric oversize taps in +0.005, +0.010 and +0.015 tolerances for worn or plated holes, and helicoil taps that cut the specific form a wire insert locks into.

Helicoil taps aren't interchangeable with a standard tap — the minor diameter is different, so a standard tapping drill and standard tap won't give you the right hole. If you need a size you can't see listed, call 01732 887334.