The minimum end gap is calculated using a general rule of .004" of an inch per inch of bore. (0.1 mm per 25mm of bore) This general rule is fine for bores from 2.5 inches to 4.5 inches. So a 3 inch bore should have a minimum ring gap of .012 of an inch.(0.3mm). Different ring manufacturers use nominal bore size to manufacture their rings and as pistons can vary in size from one manufacturer to another the minimum is often not achieved. Up to 50% above these specifications will usually be acceptable. It is more important not to be under the minimum clearance as ring seizure and breakage will occur as the rings heat up to normal operating temperature. If ring end gap is under minimum and piston to bore clearance is correct the rings should be replaced or the end gaps filed to size.