O-rings are designed to help seal fluid from entering one area of a part through another, like stopping water from leaking out of a thermostat housing or to seal an electrical connection from liquid. Replacing an O-ring isn't very difficult to do, but it can be a bit odd to figure out at first if you don't know how to do it. In this case, the project is on a hydraulic cylinder; but the process is the same for lots of O-rings, so these tips will help you to install O-rings in any application.
Place the hydraulic cylinder in the bench vise, so that the long shaft of the cylinder is secure. Unscrew the threaded collar on the base of the cylinder body, and place it to the side. Pull out the cylinder piston using your hands, and place it to the side.
Hold the cylinder piston with one hand, and place the flat-head screwdriver behind one of the O-rings on the cylinder so that the O-ring is out away from the piston. Pivot the screwdriver against the top of the piston and pull back, prying the O-ring off the cylinder.
Place the replacement O-ring on top of the cylinder, then place the flat-head screwdriver through the center of the O-ring so that the screwdriver is between the top of the piston and the O-ring body. Push the O-ring down the cylinder until it slides into the slot on the piston and is seated correctly.
Place the piston back inside the cylinder body, and hammer it back into place using the mallet. Then screw the collar back onto the bottom of the cylinder.
