§1Two jobs, one thread
Every previous page’s thread only had to hold. A pipe thread must hold and contain — and the section’s three answers to “where does the seal live?” organise everything that follows.
The first answer puts the seal on the thread itself: taper the whole thread slightly, screw it home, and the male form wedges into the female until the flanks jam metal-to-metal — the NPT and BSPT families of §4, with a smear of sealant closing what the wedge cannot. The second puts the seal on a face: keep the thread parallel and let it do nothing but clamp, while a washer, O-ring or gasket trapped between machined faces does the containing — the G and NPS practice of §5, and mechanically the honest one, since it asks the thread only what every thread can do. The third puts the seal on a cone: the flared-tube fittings of §6, where a nut squeezes tube against a conical seat and the thread never sees the fluid at all. Each philosophy is sound alone; nearly every leak this page can prevent comes from combining them — a taper male in a parallel female of the wrong system, sealant asked to rescue a face joint, a flare nut treated as if its thread sealed. Knowing which philosophy a fitting belongs to is the whole craft.
Contents§2The name is not the bore
Pipe threads inherit their names from the pipes they fit — and pipe names describe a historical bore, not any diameter you can measure on the fitting.
The british-fasteners page met this trap from the British side; it is universal. A “half-inch” pipe thread is the thread that fits half-inch pipe, and half-inch pipe is named for the nominal bore its wall thickness once enclosed — so the thread’s actual diameters sit far above the name. Measure them: G1/2 runs 20.955 mm across, 1.65× its name, and NPT 1/2 has a reference outside diameter of 0.840 in = 21.34 mm — 1.68× the 12.7 mm the name suggests. The consequence is a permanent rule of identification: a pipe thread can never be identified by measuring its diameter against its name. A vernier reading of 21 mm means “some half-inch pipe system”, and only §6’s drill — count the TPI, check taper against parallel, gauge the angle — says which. The naming survives because it is genuinely useful at the system level (every “1/2” component of one standard fits every other), and it survives as a trap because the fittings of different systems in the same trade size are, as §4’s arithmetic will show, dimensionally close enough to start on each other.
Contents§3The systems, side by side
Six designations cover essentially all threaded pipework, and the table is the map: two angles, two philosophies, and no legal crossings between the columns.
| Designation | Form | Angle | Taper | Seals by |
|---|---|---|---|---|
| NPT | taper, male and female | 60° | 1:16 | thread wedge + sealant |
| NPTF (Dryseal) | taper, interference crests | 60° | 1:16 | thread alone, no sealant |
| R (BSPT) | taper male | 55° | 1:16 | thread wedge + sealant |
| Rc · Rp | taper · parallel female | 55° | 1:16 · — | mates with R on the thread |
| G (BSPP) | parallel, male and female | 55° | — | washer or O-ring at the face |
| NPS | parallel | 60° | — | face gasket (mechanical joints) |
| Two families, never one. The 60° American systems (NPT, NPTF, NPS) and the 55° Whitworth-form British systems (R, Rc, Rp, G) share trade sizes and, at several of them, thread counts — and differ in angle and form exactly as the whitworth page warned. The one legal internal crossing is British by design: a taper R male seals into a parallel Rp female on the threads, the standard jointing pair. Everything else that “sort of fits” across the table’s halves is §6’s cautionary tale in waiting. | ||||
§4The 1:16 taper, worked
Both taper families use the same slope — one in sixteen on diameter — and its arithmetic explains the feel of every taper joint ever made up: easy, easy, easy, solid.
NPT 1/2 runs 14 TPI, so p = 25.4/14 = 1.814 mm, and each full turn advances the male cone that far along a 1:16 slope — the effective diameter grows by 1.814/16 = 0.1134 mm per turn. Made up by hand, the joint spins freely while clearance lasts and stops the moment the cones kiss; the wrench then works pure interference: the standard two to three turns past hand-tight drives a diametral squeeze of 0.227 to 0.340 mm — a genuine press fit, generated helically, that flattens the flank contact into the near-continuous seal the whitworth page’s radiused form makes possible on the 55° side, with PTFE tape or paste filling the last spiral of the 60° side’s flat-crested path (or, in NPTF Dryseal, crest–root interference filling it instead). The same numbers write the two cautions. Every extra turn adds another tenth of a millimetre of wedge, and the female fitting is a ring being burst from inside — over-tightening splits cast fittings, and “one more turn to fix the leak” is how. And made-up position, not torque, is the honest metric: a joint that reaches its turns loose, or runs out of thread before sealing, has mismatched or damaged threads that no amount of wedge will cure.
§5Sealing on a face
The parallel philosophy separates the two jobs completely: the thread clamps, a compliant element seals, and each is asked only what it is good at.
A G-thread union is the hero’s right-hand panel: parallel male, parallel female, and between the fitting faces a bonded washer, fibre washer or O-ring squeezed to a controlled crush as the thread pulls up. The virtues follow from the separation. Sealing force is set by the washer’s compliance, not by wedging strain in the fitting, so nothing is burst and nothing is galled; the joint makes and remakes indefinitely with a fresh washer, where a taper joint consumes a little of itself each cycle; and orientation is free — a taper fitting seals wherever its turns run out, while a parallel union can be aligned and then sealed, which is why gauges, unions and instrument connections live here. The costs are a machined seat to keep clean and a washer to stock and remember. The R-into-Rp pairing of §3 is the British system’s deliberate hybrid — taper male carrying the seal into a parallel female — and it works because both sides were designed for it; the same move improvised across systems, or a G male asked to seal in tape like a taper, is exactly the philosophy-mixing §1 promised would leak. On the 60° side, parallel NPS threads appear where a mechanical joint carries its own gasket — hose couplings, locknuts, fixture stems — and never as a sealing thread alone.
Contents§6Hoses, flares and the drill
Hydraulic and fuel lines add the third philosophy — the cone — and the whole page then compresses into a four-step identification drill.
A 37° flare (JIC) fitting seals where the flared tube end is crushed between a coned seat and the nut’s shoulder: the cone is the seal, the thread is a clamp, and the fluid never touches a flank — which is why flare threads take no tape, ever, and why tape on one actually causes leaks by faking the nut’s torque before the cone has seated. The 45° flare of low-pressure work and the O-ring-boss and face-seal families extend the same principle. That completes the toolkit, and the drill applies it: first, calliper the thread at both ends — different readings mean taper, equal mean parallel — and check the pipe end’s form for a flare seat, which settles the philosophy. Second, count the TPI with a gauge. Third, gauge the angle, 55° against 60°, because the counts conspire: at the half-inch trade size, NPT and BSPT both run 14 TPI — the classic pairing that engages sweetly for three turns and never seals — while at three-eighths the systems announce themselves, NPT’s 18 against BSP’s 19 TPI refusing to mesh at all. Fourth, match sealing method to system: sealant on taper threads only, washers on faces, nothing on flares. Ten patient seconds per fitting; every skipped step on this list has a named leak waiting for it.
Contents§7Quick reference
The working core of the page on one card rack.
Philosophies
thread · face · cone
never mix them
Names
NPT 1/2 OD = 21.34 mm
1.68× the name — never measure to the name
Taper
1:16 · 1.79° half-angle
0.1134 mm per turn at 14 TPI
Make-up
hand-tight + 2–3 turns
0.227–0.340 mm of wedge
The drill
calliper · count · angle
then seal to the system