Thread types in hydraulics: size charts for BSP, metric, NPT, JIC and ORFS

Two features separate the families: the thread form and the flank angle. Parallel threads (G, M, JIC, ORFS) have the same outer diameter front and back, tapered threads (R, NPT) narrow towards the tip. The flank angle is either 55° (Whitworth: BSP) or 60° (metric, NPT, UN/UNF). In the end, though, the sealing principle decides, not the thread alone: the same thread can seal over a cone, a flat face or a seal ring. How to measure an unknown thread step by step is shown in Identifying a hydraulic thread.

BSP is the Whitworth pipe thread with a 55° flank angle, the most common connection in Europe. There are two versions: G is parallel (ISO 228-1) and does not seal in the thread but over a 60° cone seat, a flat or soft seal or an O-ring at the stud base. R is tapered (ISO 7-1, taper 1:16) and seals metallically in the thread with a sealant. G and R have identical core data at the same nominal size and differ only in the taper.

The metric ISO fine thread (M with pitch, DIN 13) has a 60° flank angle and is parallel. In hydraulics it plays two roles: as the union-nut and stud thread of the DIN 2353 cutting-ring fitting, where the seal forms on the 24° cone, and as a stud thread per DIN 3852 that seals over a seal ring, a sealing edge or an O-ring. The full series with tube OD, thread and rated pressure are in the article DIN 2353: the LL, L and S series.

The classic trap in any thread comparison is NPT: the American tapered pipe thread standard (ANSI/ASME B1.20.1) looks confusingly similar to the tapered BSP thread R but is not compatible with it. NPT has a 60° flank angle and a 1:16 taper and seals in the thread by wedging the tapered flanks, usually with a sealant; the dry variant NPTF (Dryseal) seals metallically through deformation of crest and root.

With JIC the size lives in the dash: the number states the tube outer diameter in sixteenths of an inch. Behind it is the 37° flare fitting per SAE J514 / ISO 8434-2: the parallel UNF/UN thread (60°) only generates the clamping force; the seal forms metallically on the 37° flare cone. JIC is widely used in machine building and mobile hydraulics.

ORFS (O-Ring Face Seal, SAE J1453 / ISO 8434-3) seals over an elastomer O-ring in a groove of the flat face; the parallel UN thread only generates the clamping force. That makes ORFS especially vibration-resistant and suitable for pressures up to about 420 bar. Details on the design are in the article ORFS fittings explained.

The sizing basis is what matters, because it differs by family: JIC and ORFS dash mean the tube outer diameter in sixteenths of an inch, BSP and NPT port sizes the nominal pipe size, DIN 2353 the tube OD in millimetres. The following table places them side by side per tube outer diameter; the rows are aligned by the customary nominal assignment, and the DIN 2353 column is an approximation to the nearest metric tube OD.