plunger-piston

from The Century Dictionary.

• noun In a pump, a solid cylindrical piston, either operated by a special piston-rod with a crosshead, or protruding from the pump-barrel sufficiently for the direct attachment of a pitman to it outside of the pump-cylinder.
• noun The solid piston of a pressure-gage, steam-indicator, or some similar instrument. Also called plunger-lift and plunger-bucket.