The Piston Ring: Sealed by the Gas It Holds Back

Early steam pistons were sealed with packing — hemp or leather pressed against the cylinder by adjustable rings. In the 1850s John Ramsbottom, locomotive superintendent of the London and North Western Railway, replaced it with a thin split ring of iron sprung into a groove round the piston. It is still how every piston in every engine is sealed. The ring's own spring presses it lightly against the wall. The gas it is holding back gets into the groove behind it and presses it far harder — so the higher the pressure, the tighter the seal. Its one enemy is heat: the ring must have a gap so it does not jam when it grows. This rung works out both effects and makes a split ring from a pipe to feel its spring.