Half the mistakes in an order for hoses and adapters come from a thread that was read wrong. The systems look alike, some of them will even start into each other for a couple of turns, and the joint then weeps under pressure or strips the thread in an expensive manifold. What follows is the order of checks used at the counter, in the workshop and out in the field, where the only tools are whatever is in your pocket.

Identifying a thread is not one measurement but four answers. Skip any of them and you are left with two or three candidates that look almost the same.

1. Male or female. You always measure the outside diameter of a male thread and the inside diameter of a female one. These are different numbers and must not be mixed up.
2. Parallel or tapered. This decides where the joint seals: on the thread itself or on a separate surface.
3. Diameter and pitch. Pitch is given either in millimetres or in threads per inch, and that alone tells you which family the part belongs to.
4. What does the sealing. A 24°, 37°, 30° or 60° cone, a flat face with an O-ring, an O-ring in a groove, or the thread itself. This is the last and deciding check.

On a tapered thread the diameter grows from the nose towards the base. On a parallel thread it is the same along the whole length. There are three ways to tell, and the first two need no tools.

• Lay a straight edge against the thread — a rule, a knife blade, the edge of a phone. On a parallel thread every crest touches it at once; on a tapered one a gap stays open at the end crests.
• Start a mating nut by hand. On a parallel thread it runs freely the whole way; on a tapered one it binds after two or three turns.
• Measure the diameter at the nose and at the base. Half a millimetre of difference or more on a short stud means tapered: NPT and BSPT are cut at 1:16, which changes the diameter by about 0.625 mm per centimetre of length.

A tapered thread seals on its own flanks, which is why it is fitted with sealant or tape. A parallel thread never seals by itself: it needs a cone, an O-ring or a bonded washer under the shoulder. If there is a washer under your thread, the thread is parallel.

Next come two numbers: outside diameter and pitch. In a workshop that means a caliper and a pitch gauge, two minutes of work.

With no tools, take the diameter with a strip of paper. Wrap it round the thread, mark where it meets itself, unwrap it and measure the length. The diameter is that circumference divided by 3.14. The method is good to a few tenths of a millimetre, which is enough: neighbouring sizes in any system are two millimetres apart or more.

Pitch is counted off the crests. A metric thread is specified in millimetres: lay a rule against it, count ten gaps between crests and divide the length by ten. An inch thread is specified in threads per inch: count the crests over 25.4 mm. The one thing to watch is the first crest — you count gaps, not peaks.

One useful sign needs no measuring at all: British BSP threads have a 55° form with rounded crests and roots, while metric, NPT and the inch UN/UNF threads have a 60° form with flat crests. Under a lens it is a difference you cannot mistake.

A nut, in fittings, is not a separate part but a female thread, and it is measured differently. Caliper jaws set for an internal measurement give the diameter across the thread crests, which is smaller than the designation by roughly one pitch plus a little. For a metric thread of 1.5 mm pitch the difference is about 1.6 mm: measure 12.4 and you are holding M14×1.5.

An impression is quicker and more certain. Press something soft into the thread — solder, lead, a folded piece of kitchen foil. Modelling clay works too. The impression shows both the pitch and the shape of the form, and you can measure it in comfort on a bench.

Easier still if a male fitting of known thread is at hand. Start it by hand. If it runs the whole way and seats without play, the thread is the same. If it binds after two turns it is not, and it must not be forced: a stripped female thread can only be replaced.

Then look inside, because the seat gives the system away. A short chamfer deep in the bore is a 24° cone for a bite ring. A long shallow cone is 37° JIC. A steeper cone with a BSP thread is 60°. A flat face with a groove for an O-ring is ORFS. If there is neither cone nor groove and the thread itself is tapered, the sealing is on the flanks: NPT or BSPT.

One last check: turn the nut on your own fitting. A swivel nut spins freely and does not drag the tube with it — all it does is clamp. If thread and tube turn together, this is not a swivel nut but a plain male part with a nut on the body.

When there is no rule, anything with a standard size will do instead. The dimensions below are fixed by standards and do not change with the year of issue.

The most practical one is a bank card. Its long side is exactly 85.6 mm and it is 0.76 mm thick. Held against the part it gives the diameter, and its corner shows whether the thread is parallel or tapered. A coin works as a go/no-go gauge: if a male thread is clearly smaller than a 2 euro coin you are in the M14 to M22 range, or G1/8 to G1/2, and only the pitch is left to settle.

The second field method is comparison. A machine almost always has a second port of the same kind, or a mating half you can start by hand. If an unknown stud runs freely into a known nut all the way and seats without play, the systems match. If it is stiff or binds after two turns they do not, and it must not be forced with a wrench: that ruins both the stud and the port.

This is the check that finally separates systems sharing the same thread, and there are more of those than you would expect.

• A 24° cone inside the nut with a metric thread — DIN 2353, ISO 8434-1, light series L or heavy series S. It seals on the cone and the bite ring.
• A 37° cone with an inch UN/UNF thread — JIC, SAE J514. Metal to metal; sealant is unnecessary and harmful here.
• A flat face with an O-ring in a groove, inch thread — ORFS, SAE J1453. The most vibration-resistant of the common joints.
• An O-ring at the base of a straight inch male thread — SAE O-ring boss. The threads are the same as JIC, but it seals in the port, not on a cone.
• A 60° cone with a BSP thread — the familiar European BSPP seat. A 30° cone with a BSP thread is Japanese JIS B 8363; a 30° cone with a metric thread is Komatsu.
• A tapered thread with no cone and no ring at all — NPT or BSPT, sealing on the flanks.

• G1/4 and NPT 1/4. Outside diameters of 13.16 and 13.72 mm, 19 against 18 threads per inch. They start into each other, hold for an hour or two and then weep. Check the taper and the rounded crests.
• JIC 37° and SAE 45°. In sizes -4, -5, -8 and -10 the thread is identical and only the cone angle differs. The angle is the only reliable sign.
• JIC 37° and ORB. The threads match completely. Look at the end: ORB has a groove for an O-ring at the base, JIC has a cone on the nose.
• BSPP and BSPT, G and R. The marking on the part is often worn away and the threads look alike. The taper tells them apart.
• JIS 30° and Komatsu 30°. Same cone; the first has an inch BSP thread, the second a metric one. Compare the pitch.
• Light and heavy DIN series. M18×1.5, for example, appears in both L and S, but for different tube diameters. The thread will match and the tube will not.

Diameters are nominal, for male threads. A real measurement usually comes out 0.1 to 0.3 mm smaller because of tolerance and plating — that is normal, not a different size.

If you are reading this on a phone there is nothing to print: the same scales, plus pitch combs and a wedge for diameters, are on the screen gauge, which sizes itself to your display against a bank card.

Print this page, or save it as PDF, at 100 % — not “fit to page”. Check the control line first: if it measures exactly 100 mm, the rest of the scale is right too.

Send us photographs and whatever you managed to measure and we will identify the system and find the part number in the catalogue. Three shots are enough: the thread from the side, the end face straight on, and the part next to a rule or a coin. If there is any marking on the hex or the nut, photograph that too — often it settles the question at once.