My own reference material for threaded connections and thread sealing
Most scuba equipment has some form of thread on it, and this information is almost never published, however if one needs to create adaptors or new hardware to interface with existing equipment then this information is vital. Threads can be measured, so I measure and keep track of threads as I come across them.
NPT is somewhat rare in underwater equipment itself, however it is commonplace among gas blending equipment and compressor setups, it is also one of the more common threads to be machined in the home shop. A hand tight NPT mate should achieve 4-6 turns of hand-tight engagement with no thread tape. With thread tape at least 4 threads of engagement should be achieved. When using thread tape, ensure that the first thread of the fitting is clear, as thread tape pieces dangling into the gas flow can foul instruments or valves, or in an oxygen rich environment can be an ignition source.
Tightening NPT by wrench should result in an additional 0.5-1.5 thread tapes.
The relevant standard for pressure ratings and mating information is ASME B1.20.1.
Some systems, notably the poseidon rebreathers, use helicoils in their design in order to save wear on the factory threads. These are standard automotive and industrial components that use a thread of the same pitch at a larger diameter, and then include a coiled piece of rectangular wire to fill the space. This means that wear takes place on the wire insert rather than the more expensive part the insert sits in.
Loctite 567 and 577 can hide many crimes. Loctite 567 is designed for low pressure sealing, it is an anerobic cure that needs an active metal to cure properly. Loctite 577 can handle higher pressure and form seals on straight threads or straight-to-taper mates.
Neither of these thread sealants are suitable for 100% oxygen.
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| SAE/UNF | 3/8” | 24 TPI | Regulator Low Pressure Ports, BCD LPI Nipples |
| SAE/UNF | 7/16” | 20 TPI | Regulator High Pressure Ports |
| SAE/UNF | 5/16” | 24 TPI | Military High Pressure Ports |
| SAE/UNF | 9/16” | 18 TPI | Second Stage Inlets |
| SAE/UNF | 1/2” | 20 TPI | Non-Standard Regulator Low Pressure Ports |
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Metric | 16mm | 1mm | Oxygen Cell Port |
| Metric | 38mm | 1mm | P-Port |
| Metric | 33mm | 1.5mm | Sitec Drysuit Valve |
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Imperial | 1 3/8” | 12 TPI | KISS Classic Counterlung Port |
| NPSM | 1” | 11.5 TPI | KISS Sidewinder Tower port |
| Imperial | 4 1/2” | 8 TPI | KISS Sidewinder Canister |
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Metric | M6 | 1mm | Canister thumbscrews (helicoiled) |
| Metric | 50mm | 2mm | Loop Hose |
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Metric | 36mm | 2mm | Loop Hose |
Note that Megalodon breathing loop threads are right handed on the inhale side, and left handed on the exhale side.
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Metperial | 42mm | 14 TPI | Canister |
| Imperial | 2.5” (63mm) | 14 TPI | Counter Lungs |
| Standard | Diameter | Pitch/TPI | Usage |
|---|---|---|---|
| Metric | 18mm | 1.5mm | M18 Cylinder Neck |
| NPS | 3/4” | 14 TPI | 3/4” Cylinder Neck |
| Metric | 25mm | 2mm | M25 Cylinder Neck |
| G-series Pipe | 5/8” | 14 TPI | DIN valve inlet (G5/8) |
| Metric | 26mm | 2mm | M26 Valve inlet (European Nitrox) |
| SAE/UNF | 7/16” | 20 TPI | Swagelok 1/4” Tube Ferrule |