Abstract: A clean burning pre-combustion chamber system and method for modifying existing "dirty" natural gas engines to meet modem exhaust emissions standards includes a number of small, water-cooled pre-combustion chambers which can be threaded into the existing spark plug holes, of which there are two per cylinder. A specially shaped ignition chamber within each pre-combustion chamber enhances fuel flow and virtually eliminates the "knocking" which is typically found in pre-combustion chamber modified engines. The inventive system allows existing engines to be economically converted with minimal or no structural changes to the cylinders, heads or valve covers.