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 1960ís.
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.