Caps & Plugs

A cap and a plug close the same opening from opposite sides. The plug enters a female port on a male thread; the cap slips over a male end on a female thread. Where the seal forms depends on what is closed: a 37° cap on the cone, an ORFS plug on an O-ring against the flat face, an O-ring boss plug in the counterbore, a pipe plug on the taper itself.

What governs ordering is duty rather than shape. A protective closure keeps swarf, moisture and paint out during shipping; a pressure-rated closure is part of the circuit and must be sealed to the standard of the connection it stands in for — SAE J514, J1453 for ORFS, J1926 for O-ring boss, ASME B1.20.1 for NPT, ISO 228 for BSP.

Where it's specified

• Shipping and storage — a plastic closure keeps contamination out of open ports until the machine is plumbed
• Spare ports on a manifold — a metal plug sealed to the port standard makes an unused test point permanent
• Commissioning and testing — capping a branch lets the rest of the circuit be proved first
• Field repair — blanking a damaged leg keeps a machine earning until the correct run is made up

Range and materials. Plugs and caps cover JIC 37°, ORFS, O-ring boss, NPT and BSP in parallel and taper forms, in hex-head, hollow-hex, internal-hex and knurled patterns, in steel and stainless, alongside plastic protective closures. Sizes follow the dash and thread ladder of the connections they close.

Ordering notes. Match the closure to the connection, not the thread alone: an O-ring boss plug needs a straight-thread port and the right O-ring, an NPT plug depends on thread engagement and sealant, and the two are not interchangeable even where sizes seem to agree. Check the O-ring compound against the fluid, never leave a shipping cap in a pressurised line, and choose an external hex where a port gets reopened.