SeaCore is a brand of sterndrive marine propulsion systems manufactured in the United States by MerCruiser. The SeaCore engine design utilizes materials, technologies, and systems, created specifically for Mercury Marine, to prevent galvanic corrosion within its engine, transom and drive. SeaCore propulsion includes models generating between and horsepower (&#;kW). SeaCore is designed for a wide variety of vessels operated in or moored on saltwater.

In , the company also introduced its first large outboard, the approximately cubic inch, cylinder in line, "Thunderbolt". This engine was rated at horsepower, but actually put out around . There was soon an "H" version designed for high rpm use. This version put out nearly horsepower, while being rated at "+". The Thunderbolt engine benefited from upgrades and updates, eventually becoming the Mark , Mark , and Mark engines of the mid and late fifties. All of these were rated quite accurately at horsepower. The Mark H (high speed) developed more, but carried no rating.

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In Mercury introduced cc and cc versions of the Sno-Twister and both were dominant at the track. saw a change in the Sno-Twister, this time cc, cc, and cc versions were all introduced. These were radical for their time and were the result of Mercury Sno Pro sleds developed and raced the previous year. These sleds were featured water-cooled Kohler engines. The chassis were small, lightweight, very low profile, with curved handlebars and seat cushions that were designed for left turns (oval tracks).

In Kiekhaefer started testing at a Florida lake he called "Lake X" to keep the location secret. Later that year the Kiekhaefer Mercury company designed a new inline -cylinder, cubic inch, &#;hp (&#;kW) engine named "Mark ". Like its - and -cylinder brethren, the Mark featured internal reed valves. The reed valve blocks served as intermediate main bearings. Because the valves occupied space already necessary for the main bearings instead of mounting to an extension of the crankcase, crankcase volume was minimized, resulting in a higher crankcase compression ratio, and more power in proportion to displacement than the competition.