The plungers (6 in Figure 7) are the two smaller diameter shafts that are connected to each side of the piston. The attachment point is inside of the hydraulic cylinder. The other ends of the plungers extend into the left and right high pressure cylinders. Seals are placed around the plunger shaft to keep oil from seeping into the water side of the pump, and vice versa. The plungers are made out of either stainless steel, or, more recently, ceramic. Ceramic is used because of its ability to handle heat and high pressure with little thermal expansion.