In certain embodiments, a unique method and pre-combustion chamber (PCC) structure may ensure very efficient flame propagation of lean fuel-air mixture in natural gas engines by reducing the amount of fuel admitted to the PCC. A PCC may include an enclosed volume of 1-3% of the main combustion chamber volume, with a spark plug and a fuel passage located opposite one or more PCC discharge nozzles to create a relatively richer fuel-air mixture with relatively lower turbulence in the spark plug region and a relatively leaner fuel-air mixture with relatively high turbulence in the nozzle region, which can be reliably and efficiently ignited, resulting in a high velocity flame jet/torch emerging from the prechamber into the main chamber. The PCC may be threaded with a 22 mm×1.5 or ⅞″-18 thread size, to allow the PCC to be screwed into a cylinder head in place of a spark plug.