Help on water pumps used in Mercruiser.

Vintages on water pump housings;

The upper housing has been unchanged for quite a long time.  The most common tops fit the #1 unit 1972-1985, followed by the  Alpha I housing, from 1985-1992. The top housing fits both, and even prior to the #1 unit back to the 1960’s.

The main difference between #1 and ALPHA in the pump is the base plastic housing.  The base differs mostly in the circular part that drops into the lower unit around the shaft.  The #1 has a sharp ring around the round area of the base  with a break-space on one side. The alpha has a flat circular part and it has a larger diameter.  Both have two base seals.

The matching drive shaft which passes through, is distinguished by the pin on the top of the #1,notably missing on Alpha I type.

After 1992 there is no base, and the upper housing  is S.S.. These are not usually replaced during service.  The matching impeller has a longer blade design for more volume but lower pressure.