A hydraulic cylinder is a mechanical actuator used to provide unidirectional force through a unidirectional stroke. Hydraulic cylinders are used in almost every industry and are available in a variety of onfigurations. The two main types of hydraulic cylinders are Tie-rod and Welded. The bore is how fat the cylinder is. It's the inner diameter of the cylinder housing which is also equal to the diameter of the piston inside of the cylinder (note the piston is not the piston rod which moves in and out of the cylinder). ... Bore sizes are generally standardized numbers like 2", 2.5", 3", 3.5", and 4".