Spark plug set 816761Q4 is for use on V8 MerCruiser (LH) engines (305 and 350 CID) with Thunderbolt IV ignition systems (1981 through 1992).The Quicksilver brand is the leader in delivering uncompromising engine oils, parts and accessories to the marine industry for over 65 years. Quicksilver is the premier supplier of Mercury Marine Original Equipment replacement engine parts, oils, and accessories, but also works to serve all Original Equipment brands in the marine and powersports industries. It has earned a loyal following by providing the highest quality products that are tested through some of the most rigorous environments and testing procedures. No aftermarket company comes close to the level of research and design as Quicksilver; buying Quicksilver is buying performance, protection, and peace of mind.