Swaging — Dutch *optrompen* — is widening the end of a pipe so that a second pipe of the same outside diameter can be pushed into it. The widened end is called a socket or spigot socket.
It is one of the oldest ways of joining two pipes without a separate fitting.
How it is done
- By hand. With swaging pliers or a lance-shaped swaging iron — traditionally a plumber’s tool — the pipe end is gradually expanded. This works well on soft materials such as lead, zinc and copper.
- By machine. A pipe-end forming machine or expander presses a tapered mandrel into the pipe. This gives a far more consistent result and is the workshop standard.
Where you meet it
- Zinc rainwater downpipes. The classic case: the upper pipe slides into the swaged socket of the lower one, so water always runs the right way.
- Copper pipework. A swaged end can serve as a solder socket, saving a fitting.
- Thin-walled steel tube in services and light structural work.
An important limitation
Swaging on its own generally does not produce a genuinely tight joint. The socket surrounds the pipe, but the seam itself is not watertight. That is fine for a gravity drain with adequate overlap, but not for a pressurised line.
Rules of thumb:
- Under pressure? Only swage if you then solder, seam or press the joint. Never as a bare push-fit.
- For gravity drainage? Always put the upper part inside the lower and keep a generous overlap — on a rainwater downpipe, at least several centimetres.
- The material has to give. Hard-drawn or thick-walled tube will split at the edge. Copper swages much better after annealing.
- The wall gets thinner. Expanding reduces the wall thickness. Do not expand further than the material can take.
- Deburr before and after; a burr tears out unevenly.
Related terms
- Rainwater drainage
- Movement joint
- Facade renovation
Learn more about building terms in our knowledge base at fredsdiyplans.com.