During an engine rebuild, one of the most important considerations is overhauling the pistons and replacing or cleaning the piston rings. The piston rings need to be in perfect condition in order to maintain a seal between the piston and the cylinder bore. Piston rings endure an enormous amount of heat and pressure, along with repeated flex and exposure to carbon build-up. Piston rings and piston-ring grooves must be clean in order to allow a proper seat. Paying attention to the proper care and maintenance of the rings and grooves will ensure a professional engine rebuild.
Remove the piston rings from the piston with an appropriate expander tool. Place the tool ends between the ring gap and slowly squeeze the handle until the ring expands. Slip it off the piston. Remove all rings, the expansion ring, compression rings and oil ring and place them in a tub filled with engine solvent. Carburettor cleaner or lacquer thinner will work instead of solvent. Piston Kleen has been developed specifically to remove built-up carbon on rings and pistons. Let it soak for 24 hours.
Remove the pistons from the solvent bath. Use a paint scraper to remove built-up carbon on the top of the piston. Be careful not to gouge the piston metal. Use a brass-bristle wire brush to finish cleaning the top of the piston and sides. Use a long-bristle parts brush to clean the underside of the piston skirt and the piston pin bore.
Place the piston in a vice, top side up. Use rags between the piston skirt and vice jaws. Read the directions on the use of the piston-groove cleaning tool. Pick out the correct size bit that will cut the lands grooves in the piston---some have metric measurements, while others have standard sizes.
Encircle the piston with the groove cleaning tool and set the bit in the groove. Adjust the tension knob. Rotate the tool 360 degrees around the piston top. Examine the groove after a few rotations---the groove, or landing, should end up shiny and free of any carbon. Cut each ring groove and change the bit diameter if required to do so for the oil ring groove. Re-clean the grooves with solvent and a brush to remove all residue.
Collapse one ring and place it inside the cylinder bore. Place a feeler-gauge blade between the ring ends. Measure the end gap of each ring this way. Make sure all ring-end gaps meet required specifications; refer to the owner's manual. Use a flat file to shave away excess metal on the end of a ring if its gap measure is too narrow, according to specifications.
Reinstall the rings back onto the pistons. Use the expansion tool to open the rings up and slide them down over the piston skirt. Start with the oil ring first; then install the other two rings.
Before installing the pistons back into the cylinders, check them with an outside micrometer for out-of-rounds. Refer to the owner's repair manual for the correct measurement.