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Changing To Counter-Rotation

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If you have a boat with twin Pre-Alpha or Alpha One drives, 1972 - 1990, and are considering having a counter-rotation drive on the Port engine, here is what must be done :

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The complete lower half of the drive must be replaced. ( see picture below )

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The linkage on the Port drive must be switched at the forward control lever ( cockpit ) to make it shift into reverse when the lever is placed in forward. 

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The propeller must be changed  to a left hand prop.   

Most boats, with twin drives, made after 1991, and have Alpha One/Generation II drives, already come with a counter rotation drive on the Port ( or left hand side of the boat when viewing it from the rear ).

This is a complete counter-rotation lower unit from MerCruiser. It is p/n 1623- 8951A33

Counter-rotation lower units are only manufactured by MerCruiser. There are no aftermarket replacements for these units. These lower units have 13 teeth on the top of the vertical shaft and will not fit older units made before 1970 which have 8 teeth.

This is a completely different lower housing than used on a standard rotation drive. You cannot use your old housing and replace the internal parts because all of the internal sizes are different. This unit has different gears, bearings, seals and special thrust bearings which allows the unit to run in reverse while moving the boat forward 
( with a left hand prop installed ).

A counter-rotating lower unit does not have any in & out end play on the propeller shaft. A standard rotation unit has between 1/16 to 1/8 of an inch of in & out end play.

         : Order  # 1A33

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