NPT, NPSM and SAE hose fittings: taper, swivel, flare and O-ring boss

American hydraulics uses five hose-end families that share inch sizes and get swapped for each other with depressing regularity: NPT, NPSM, SAE 45° flare, SAE inverted flare and SAE O-ring boss. The way to keep them straight is the same as for every other family — ask where it seals — and each of the five gives a different answer.

/01Five families, five seals.

/02NPT and NPSM: same thread form, different job.

NPT tapers along its length, so the male wedges into the female and the thread flanks themselves make the seal — which is why NPT needs a sealant and why it can only be made up once or twice before the thread distorts. NPSM uses the same 60° thread form and pitch cut straight, so it cannot seal on the thread; our NPSM male swivel seals on a 60° cone instead. On hose assemblies the common pairing is an NPSM female swivel on the hose screwed onto an NPT male adapter, where the swivel's cone seals against the NPT male's 30° chamfer: no sealant, and it can be broken and remade.

/03SAE 45° flare and inverted flare.

SAE 45° flare is the older cousin of JIC: a 45° cone instead of 37°, on UN/UNF threads from 7/16-20 to 1-1/16-14, common on refrigeration, fuel and low-pressure lines and on older machinery. Its threads overlap JIC's in several sizes, so a 45° female will start on a 37° male and leak. Inverted flare moves the 45° cone inside the fitting body and threads a male tube nut into it — the arrangement on brake and fuel lines — in our range from −03 to −10, straight, 45° and 90°. Our flare ends are listed to SAE J512.

/04SAE O-ring boss.

An O-ring boss port has a straight UN/UNF thread and a spot face; the male fitting carries an O-ring in a groove under its shoulder, and the threads bottom out as the O-ring is squeezed into the port's chamfer. It is the standard port on American valves, pumps and cylinders and one of the most leak-tight connections there is, provided the O-ring is new and the right size. Our O-ring male and male boss swivel hose fittings run −04 to −24 to SAE J1926 and ISO 11926; the braided and spiral crimp ranges also carry O-ring boss male ends.

/05Stainless versions.

For corrosive and washdown duty, our 316L range lists an NPT male pipe straight and NPSM swivel female 60° cone fittings — straight, 45° and 90° — from 1/8" to 2", passivated rather than plated. A double-hexagonal NPSM swivel female adds a second hex for wrenching. The thread rules are identical; stainless threads gall more readily, so use an appropriate anti-seize on the threads and not on the seat.