It is assumed at this stage that the pistons are properly fitted to their connecting rods, and that the clearance of the piston ring to groove is checked. The end gaps should be verified by nature before the rings are installed on the pistons by fitting the rings into the cylinder bores. Place the piston assemblies and the new ring sets on a clean work surface to leave the parts together during and after end gap checking with the crankcase placed on its side to reach into the top and bottom of the bores. Install the No. 1 piston top ring and install it into the top of the bore with the No. 1 piston as a ram to press the ring down the bore to the bottom and has to be perfectly square by pressing hard against the piston crown. Check using feeler gauges to check the clearance between the ends of the piston ring and make sure that the specific blade fits through the clearance with a minimum of resistance and repeat the process with all twelve rings. When new rings are being fitted, it is not likely that the end gaps should be too narrow, but in case a measurement is discovered to be undersize, it should be altered so that the ends of the ring are not in contact with one another during operation and thus run the risk of damaging the engine. This is corrected by slowly rubbing the ends of the ring, remembering to put the rings and piston assemblies in the proper sequence. When all the end gaps of the piston rings have been confirmed they may then be installed on the pistons, beginning with the lowest ring groove (oil control ring) first, and then upwards, with each oil control ring having two side rails, separated by an expander ring, and the two compression rings being cross-sectionally different and to be seated into their correct groove and orientation with the help of a piston ring installation tool. When fitting the rings, make sure that the marks marked on one side of each compression ring, which indicates the upper facing surface, are facing upward, and the gaps at the end of the rings around the piston are spaced at 120-degree intervals to each other, and that there are no special fitting instructions given to the user by the piston ring manufacturer.