Short answer: a BSPP (G) thread seals at the fitting's face, not in the threads. Clamp a bonded seal or a captive O-ring between two machined flats and let the geometry set the squeeze at 20-30% of the rubber's thickness. ISO 228-1, the standard that defines the G series, says the same thing in its own title: pipe threads where pressure-tight joints are not made on the threads. Fit the seal to the port and the joint is the most dependable one on a washdown panel. Leave the flats bare or stuff tape into the threads and the leak path is a helix that pressure will find. The sections below give the thread sizes, the quarter-turn arithmetic that lands the squeeze, and the port features to put on the drawing.
How does a BSPP thread seal?
A BSPP thread seals at the fitting's face, never in the threads. The G profile is a parallel cylinder, so an unsealed joint leaves a helical leak path. The seal is a rubber ring crushed between two machined flats by the fitting's shoulder.
The thread itself cuts a 55° Whitworth form on a constant-diameter blank: no taper, no wedge, no interference between the flanks. Because nothing grips, the crests cannot make a joint, and pressure standing in the clearance follows the helix like a screw. The design instead concentrates the sealing where the metal is flat. The male fitting carries a shoulder or flange just behind its thread, and the female port offers a machined face square to the thread axis. Tighten the two together and the trapped rubber, not the thread, does the sealing — which is why a G joint survives vibration that walks a taped taper loose.
ISO 228 sets the sizes in the table; they are the crests you meet on fittings, glands and instrument ports.
| Size | Male crest diameter | Threads per inch | Pitch |
|---|---|---|---|
| G1/16 | 7.73 mm | 28 | 0.907 mm |
| G1/8 | 9.73 mm | 28 | 0.907 mm |
| G1/4 | 13.16 mm | 19 | 1.337 mm |
| G3/8 | 16.66 mm | 19 | 1.337 mm |
| G1/2 | 20.95 mm | 14 | 1.814 mm |
| G3/4 | 26.44 mm | 14 | 1.814 mm |
| G1 | 33.25 mm | 11 | 2.309 mm |
Crest diameters and pitches per the ISO 228 dimension tables for the G series.
What does ISO 228 actually specify?
ISO 228-1 specifies the thread form, the crest diameters and the tolerance classes. That is a 55° Whitworth profile on a parallel blank, with G1/2 running 14 threads per inch. It leaves the seal, the pressure rating and any leak test to the fitting and the face you build.
The standard's title carries the answer: pressure-tight joints are not made on the threads. What it specifies is geometry: the form, the diameters, the pitch, and two tolerance classes (A and B) for the male thread, where A runs the tighter fit. It specifies none of the sealing: no seal part, no pressure figure, no leak test. The pressure you can run is the fitting maker's published rating for the assembled joint, built around the seal that fitting was designed for. That is also why the thread size alone never tells you what will seal to what — the seal belongs to the fitting family, not to the G series.
For ordering you need two numbers off the table above, the crest diameter and the pitch, so you never need to buy the standard to buy the right washer.
Bonded seal or O-ring: which one fits your port?
Fit a bonded seal when the male fitting lands on a flat machined boss. A bonded seal is a steel washer with a rubber ring bonded inside its bore. Where the port has a recess, fit a captive O-ring in a machined groove; a copper washer covers the high-temperature corners.
The bonded seal is the default on BSPP hardware. The steel washer slides over the male thread and seats on the unthreaded flat at the fitting's shoulder. Tightened, its rim bottoms on the boss and traps the rubber at its designed squeeze — the steel is what stops the soft ring from splaying into the thread clearance. Order it by thread size as a bonded sealing washer for G1/2: the bore runs just clear of the crest, about 22 mm on that size. The lookalikes fail: a plain fiber washer has nothing elastic to compress, and stacked rubber-and-metal rings squeeze unevenly.
Where the port gives you a machined recess instead of a flat boss, fit a captive O-ring on the flange and size the groove to the squeeze band. The rectangular groove design guide carries the groove math, and the face-squeeze article carries the compression targets for a sealed enclosure face. One more family to keep apart: fittings that seal on a machined 60° cone in the nose look like face-seal parts and are not. The O-ring face seal versus 60° cone sealing comparison sorts the two apart.
How much squeeze does the rubber need?
Aim for 20-30% compression of the rubber ring. On a 2 mm ring that is 0.4-0.6 mm of crush, which for a G1/2 fitting at a 1.814 mm pitch is what about a quarter turn past first contact delivers.
The band is static-face seal practice. Below about 10% squeeze the ring bridges the machining marks and weeps at low pressure; above about 40% nitrile takes a permanent set, so the next assembly starts with a flat ring. Keep the crush between the bounds and the ring follows the surface and rebounds for the next opening.
Worked example: a G1/2 fitting on a flat boss, sealed with a bonded sealing washer whose ring cross-section is 2.0 mm (a typical figure; take the real one from the seal drawing):
- Band: 20-30% of 2.0 mm = 0.4 to 0.6 mm of crush (2.0 × 0.20 = 0.4; 2.0 × 0.30 = 0.6).
- Pitch: G1/2 runs 14 threads per inch, so one turn advances 25.4 mm ÷ 14 = 1.814 mm.
- A quarter turn advances 1.814 ÷ 4 ≈ 0.45 mm.
- Squeeze: 0.45 ÷ 2.0 = 22.5%, inside the band.
So the habit that lands the squeeze without a torque wrench: run the fitting hand-tight until the rubber first takes, then a quarter turn with a wrench on the 14 TPI sizes (G1/2, G3/4). Finer pitches move the number. On G3/8's 1.337 mm pitch a quarter turn gives 0.33 mm, 17% of the ring and under the band — turn about two-fifths of a revolution there (1.337 × 0.4 ≈ 0.53 mm, 27%) or fit a slightly thicker ring.
Elastomer choice sits beside the squeeze, because the fluid sets which rubber survives the duty:
| Elastomer | Usable range | Choose it for |
|---|---|---|
| Nitrile (NBR) | about -30 to 120 C | oils, fuels, hydraulic fluid |
| Neoprene | about -40 to 120 C | weathering, refrigerant duty |
| EPDM | about -50 to 150 C | hot water, steam, caustic washdown |
| Silicone | about -60 to 200 C | dry heat; weak against hot caustic and abrasion |
Ranges are typical O-ring handbook values; the datasheet of the named seal wins.
Where does the sealing face sit on the enclosure?
The female half of the joint is your job: the port needs a machined flat face — a boss or a spot face square to the thread — or a counterbore with an O-ring groove. A bare punched hole in thin sheet gives the seal nothing flat to land on.
On a sheet wall the thread is the easy part and the face is the hard part. On, say, a 1.5 mm stainless wall, a G1/2 thread picks up 1.5 ÷ 1.814 ≈ 0.8 turns of engagement, and the sheet cannot carry the clamp load that crushes the seal. Two details work: a bulkhead fitting through the wall, one sealing washer on each side, locked by the fitting's own nut, so the fitting pair carries the clamp; or a boss with a full-length thread and a machined face for the washer to land on. A gland port is the same joint in a bigger size — the G3/4 cable gland sizing article covers those ports and their fittings in depth.
The cutout designer lays the port, the counterbore and the spacing onto any face of a catalog or custom box and sends the layout with the quote. Fully machined bosses go through the custom stainless enclosures page, and the cutouts, glands and windows guide collects the port basics.
One rating rule closes the list: an assembly is only as tight as its lowest-rated penetration. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. A fitting that its maker rates below IP66 pulls the whole box down to that figure, and IP66 is the powerful-jet test — a 12.5 mm nozzle at 100 L/min — so read the fitting datasheet for that code before you bolt it in.
Can PTFE tape or pipe dope seal the threads instead?
Sealant belongs on tapered threads. An R or NPT joint wedges as it tightens and the paste fills the residual spiral, while a G thread offers a parallel clearance that pressure simply pumps the filler out of.
A doped G joint can hold low-pressure air for a season, which is exactly what makes the mistake durable. Paste in the crest clearance is not compressed by any metal, so every pressure cycle and every temperature swing pumps it a little further out of the helix. The joint weeps months later with no visible fault, and the fix is to rebuild it with the face seal it wanted. Face seals also hold vacuum, where a doped taper creeps.
Two notes that save a retrofit. A wipe of grease on a G thread protects it at assembly — an assembly aid, not the seal. And chasing a weeping face joint with more wrench distorts the boss, costing you the flat face the seal needs. When the machine in front of you has NPT holes and G-thread hardware, the sealed-adapter pairing covered in the G thread versus NPT gland article keeps the two thread families honest.
What do you specify on the order?
Put four things on the drawing: the thread (G1/2, ISO 228), the seal (a bonded sealing washer bored just clear of the crest, or an O-ring groove), the elastomer (nitrile to 120 C, EPDM to 150 C), and the machined face.
- Thread: write the letter — G1/2, ISO 228 — because a 1/2 NPT callout orders a different port, a different tool and a different seal.
- Seal: bonded sealing washer for flat bosses, captive O-ring for grooved ports; keep the squeeze in the band above.
- Face: a spot-faced boss or counterbore, drawn square to the thread axis; the flat rubber gasket specification guide gives the durometer and compression figures when the joint uses a soft flat gasket.
- Position: ports low and on the hinge side shed hose water and keep harness strain off the seal.
- Spares: one fresh sealing washer per port size taped inside the door — a used washer is a spent washer.
| Port in front of you | Order this | The number that checks it |
|---|---|---|
| Machined flat boss, G fitting | Bonded sealing washer | Metal contact plus a quarter turn = 22.5% squeeze on a 2 mm ring |
| Counterbore with an O-ring groove | Captive O-ring | 20-30% squeeze set by groove depth |
| Oil or fuel at the port | Nitrile washer | good to about 120 C |
| Caustic washdown, hot water, steam | EPDM washer | good to about 150 C |
| Dry heat past 150 C | Silicone washer | good to about 200 C, weak in abrasion |
| Thin sheet, no boss | Bulkhead fitting, washer each side | assembly rating = the fitting maker's rating |
Send the port layout with the quote through the cutout designer, or a full machined-boss drawing through the custom stainless page. Either way the enclosure is delivered in 10 business days, counted from your order — from your approval of the drawing on custom work — anywhere in the world.
Decision, one line per case: a flat boss takes a bonded sealing washer, a grooved recess takes a captive O-ring, thin sheet takes a bulkhead fitting with a washer on each side, and the tape stays in the drawer for tapered threads.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Why does a G1/2 thread measure 20.95 mm when it is called half inch?
The figure names the nominal bore of the pipe the thread was cut for, not the thread. ISO 228 fixes the crest diameter: G1/2 measures 20.95 mm across the crests and G1 measures 33.25 mm. Order seals by the letter and the size — a G1/2 sealing washer — rather than by the inch figure alone, because a 1/2 NPT thread shares the nominal size and nothing else.
Can you reuse a bonded sealing washer after opening the joint?
Replace it. At 20-30% squeeze the rubber takes a permanent set in the crushed position; a used washer arrives flattened and gains almost no new squeeze when you retighten, so the metal stop lands on a flat ring with little pressure at the seal. Tape one fresh washer of each port size inside the door and treat every opening as a new seal.
Which elastomer survives caustic washdown at the port?
EPDM. It handles caustic, hot water and steam from about -50 to 150 C, which covers most washdown lines. Nitrile wins in oils and fuels but tops out near 120 C, neoprene runs about -40 to 120 C for weathering duty, and silicone reaches about 200 C yet tears in abrasion and weakens in hot caustic. Match the fluid first, the temperature second, and the named seal's datasheet last.
How do you tell a BSPP port from an NPT port before ordering fittings?
Measure the taper. A G thread runs parallel, so calipers read the same crest along the whole thread; an NPT port narrows at 1:16, about 1.6 mm across a 25 mm run of thread. At the 1/2 in size both cut 14 threads per inch, so thread count alone cannot separate them — calipers over a few turns settle it in a minute.
Does a bonded-seal fitting keep a NEMA 4X / IP66 enclosure rating?
Only as high as the fitting itself. IP66 is the powerful-jet test, a 12.5 mm nozzle at 100 L/min, so a bulkhead fitting that its maker rates IP66 keeps the box there, while a lower-rated fitting pulls the assembly down to its own figure. Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the lowest-rated penetration sets the assembly's figure.
