[cleszkie]

Your picture shows a plug with a tapered seat. Make sure this is the correct style plug for your heads. There are two styles - the first has a tapered seat like yours. It requires the equal an opposite countersink machined in the head for it to seal correctly. The second style has a flat seat with a compression washer. It requires a flat mating surface on the head for it to seal correctly. If you mix and match the plug and head types you may get combustion chamber leaks. It would also foul the outside of a spark plug with exhaust soot very much like your plug has on it. Just a though.