For the complete engine maintenance story, start with the first article: Periodic Engine Maintenance and Spares.
Knowing which engine you have does not confirm which water pump you have.
Universal Medalist used both Sherwood and Oberdorfer water pumps throughout the production of their diesel engines. After Medalist was purchased by Westerbeke the pump was changed to a different Sherwood pump. After this change, we are told that all Universal diesels installed in Catalinas used the later Sherwood pump.
For those who need to know every detail in order to make a decision, we have posted a lot of information on our site regarding engine models, years, impeller dimensions etc. One other factor adds additional confusion. Even if you have an early boat, you may have the new Sherwood pump. As the old pumps wear out Westerbeke offers an upgraded replacement retrofit kit. This kit replaces either the old Sherwood or Oberdorfer pump with the new Sherwood pump. A previous owner may very well have upgraded to this pump in the past. All this information makes it looks intimidating and confusing to determine what impeller you need.
It's easy to determine which pump you have by simply looking at it!
Refer to the drawing at right. You will notice the housing is secured to the pump with three screws. This is the late Sherwood pump. For this pump you need Engine Spares Kit #Z2555 or impeller kit #Z2545.
Looking at this drawing you will notice the cover plate is secured to the pump with six screws. This is the early Sherwood pump. For this pump you need Engine Spares Kit #Z2554 or impeller kit #Z2549.
So it's as easy as three, four, or six mounting screws. Just click on the part numbers above and you will be taken directly to the order page.