Push-in fittings and air tube: getting the sizes right

Push-in fittings are the quick connectors on air lines: push the tube in and it holds; press the ring and it comes out. They are cheap, but a wrong size means a second trip to the market. Three things to know.

1. Tube size is the outside diameter

Metric air tube is sold by its outside diameter: 4, 6, 8, 10 and 12 mm are the common sizes. Inch tube (1/4″, 5/16″…) looks almost the same but does not seal in a metric fitting. Do not mix them.

2. The thread on the other end

Most fittings in Iran have BSP threads (written R or G): 1/8, 1/4, 3/8 and 1/2 inch. American machines use NPT, which looks very similar but does not seal with BSP. Small valves often use M5. If you are not sure, take the old fitting with you.

3. Polyurethane or nylon tube

• Polyurethane (PU): soft and flexible, easy to route around a machine. The usual choice.
• Nylon (PA): stiffer, takes more pressure and heat, keeps its shape on long straight runs.

Two tips

Cut the tube square with a tube cutter, not a side cutter: a slanted cut leaks. And push the tube all the way in, past the first click, until it stops.

What to tell the seller: tube size, thread size and type, the shape (straight, elbow, tee) and whether you want a brass or plastic body.