reversing-cylinder

from The Century Dictionary.

• noun The cylinder of a small auxiliary steam-engine used to move the link or other reversing-gear of a large steam-engine, when the latter is too large to be quickly and easily operated by the hand: now much used in marine engines.
• noun A small cylinder connected to the head of the steam-cylinder of the air-compressor for an air-brake system.
• noun A cylinder to aid in the reversing of the bed of a printing-press.