Hello Mick. Spark plug seals (the ones that press fit into the cylinder heads) were not used in the factory. I know several companies that peddle them to Hemi owners claimming that they offer better sealing against oil pouring into the combustion chambers. I do not know if this is a problem as my own engine is not running yet. It is my opinion that this part is most often used by hot-rodders rather than restorers. Their engines are suped for racing or street performace which may or may not require these extra seals. The spark plugs screw in far enough so that there should not be oil seepage in the combustion chambers during normal operation of the engine. I would like to hear from members who may be using these seals as to their usefullness. Bye for now. Doug 58 Crown coupe