Plunger is the main part of mud pump fluid end. The discharge pressure of mud pump is generated by the up-and-down motion of the piston in the cylinder. Small piston---high pressure and low flow rate. Big piston---low pressure and high flow rate. Metric pump and inch pump adopts metric piston and inch piston respectively. Generally，φ170，φ160，φ150 and 6'', 6-1/2'', 7'', etc. is used.