How to measure a hydraulic fitting

1. Thread diameter, with a caliper

Put the caliper across the thread crests of a male fitting, or inside the crests of a female port. Take the reading at the first full thread from the open end, square to the fitting axis. Not over the seat, not over the hex, not over the lead chamfer - all three read wrong and all three are the usual reason an identification comes out wrong.

Read millimetres if you can. Most of these standards sit a few tenths of a millimetre apart, and tenths are easier to hold in your head than thousandths of an inch.

What you read will be a little under the published basic diameter, and that is correct rather than a fault. The class allowance and the tolerance band both take metal off the crest, and plating, wear and a caliper riding on a crest all pull the same way. A reading meaningfully OVER basic is a different, larger fitting.

• A reading that lands between two sizes usually means the jaws are on a chamfer or a damaged crest. Move back a thread and take it again.
• On a female port, measure across the crests inside the bore, not across the counterbore or the spotface.
• Pipe threads are not named after anything you can measure. A 1/4 in NPT reads about 13.7 mm; subtract roughly 6 mm from a pipe-thread measurement to get its trade size.

2. Threads per inch, with a pitch gauge

Seat a thread pitch gauge against the crests until a blade sits flat with no rocking. If a blade rocks, it is the wrong pitch or the wrong form, and both of those are useful results.

With no gauge, count crests across one inch with a rule. On a fine thread, count over half an inch and double it. One thread per inch is a real difference, not a rounding error: at 1 1/16 in diameter, 12 threads per inch is a -12 JIC and 14 threads per inch is a 3/4 in tube SAE 45 degree flare. Nothing else separates those two.

The gauge also tells you the flank angle, which is a separate question from the count. A 60 degree blade will not seat on a 55 degree Whitworth thread and vice versa. That single check separates the whole American family from the whole British one.

3. Parallel or tapered

Measure across the crests at the first full thread, then again at the last full thread before the hex. A parallel thread reads the same at both ends. A tapered thread reads smaller at the entering end.

Both tapered families here run a 1 in 16 taper, which loses 1.59 mm of diameter over every 25.4 mm of thread length. Over six threads at 14 threads per inch that is about 0.68 mm - small, but well inside what a caliper resolves. Laying a straight edge along the crests shows it too.

This is the check people skip, and it is the one that separates NPT from NPSM and BSPT from BSPP. A tapered male wound into a parallel port feels tight, holds on the bench, leaks in service, and bells the port out from the inside while it does it.

4. The seat

Look straight into the end of the fitting and decide what the sealing surface is. There are only eight possibilities across every standard on this site: a 37, 45, 30 or 24 degree cone, a flat face with an O-ring groove, an O-ring on a plain shoulder behind the thread, a flat shoulder for a bonded washer, or no sealing surface at all.

On a used fitting the seat is the most reliable feature there is. It is machined once and never re-cut, while thread crests wear, get plated, and get dressed with a file by somebody in a hurry.

It is also the check that resolves the cases measurement cannot. A -08 JIC, a -08 SAE O-ring boss and a 1/2 in tube SAE 45 degree flare are all 3/4-16 UNF. Only the seat tells them apart, and getting it wrong produces a joint that assembles, holds pressure on the bench and weeps in the field.

• 37 degree cone: JIC, and AN.
• 45 degree cone: SAE 45 flare - visibly blunter and shorter than a 37.
• 30 degree cone: JIS on a BSPP thread, Komatsu on a metric thread.
• 24 degree internal cone: metric DIN 2353, both series.
• Flat face with a groove: ORFS.
• O-ring on a shoulder behind the thread: SAE O-ring boss.
• Flat shoulder for a bonded washer: BSPP.
• Nothing at all: NPT, NPTF and BSPT, which seal on the thread itself.

When two answers are still standing

Sometimes all four checks agree with two different fittings, and the honest answer is to say so. That happens whenever two standards share a thread and a seat - AN and JIC, for instance, which are dimensionally the same joint held to different specifications.

When it happens, the difference is in the part marking, the material or the finish rather than in anything a caliper can reach. The identify flow on this site lists both candidates and names the remaining difference rather than picking a winner on a margin narrower than the measurement.