Abstract: A method of installing spark plugs in a diesel internal combustion engine during conversion of the engine to a spark ignited internal combustion engine using gaseous fuel works with an engine originally having a cylinder block, a cylinder head cooperable with the cylinder block, a rocker box cooperable with the cylinder head and a diesel fuel injector for each cylinder of the cylinder block originally intended to inject diesel fuel into said cylinder. The method involves: removing each diesel fuel injector and replacing it with a spark plug adapter sleeve with a spark plug communicating with the cylinder from which the respective injector has been removed; fixing a plurality of extension sleeves to the rocker box, each extension sleeve being associated with one spark plug adapter sleeve; and providing a seal ring for sealing between the inner end of each extension sleeve and the outer end of the associated spark plug adapter sleeve when the rocker box is fixed on the cylinder head.