Axial piston

An axial piston was a droid component. 3PO-series protocol droids had axial pistons in their necks. In 10 ABY, C-3PO believed that his axial pistons had not being quite right since he had been reassembled by Chewbacca after an encounter with stormtroopers on Cloud City seven years previously.

Sources

• Dark Empire Sourcebook