† piston springn.Obsolete a spring around or inside a piston head, which acts as packing, or which presses the piston head against the cylinder wall in order to prevent leakage.
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1796 J. Robison in E. Robinson & A. E. Musson James Watt & Steam Revol. (1969) 140 The piston springs should be bent downwards at the end next the piston rod.
1881 Manufacturer & Builder July 145/3 The auxiliary pistons are provided with steel piston springs.