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Water Pump, 1/2" Oberdorfer, Repl for 1/2" Sherwood

Part #: Z3936
Many owners whose engines were originally equipped with Sherwood pumps have chosen to retrofit Oberdorfer pumps as a replacement. Easier access to the impeller for maintenance and lower cost of the Oberdorfer pump are the primary factors in this decision.

Realizing there is an opportunity in the marketplace, Oberdorfer has machined the above pump specifically as a replacement for the Sherwood, eliminating the need to modify one yourself. They charge more for this additional step, but you still save over $100.00 compared to the Sherwood pump.
Owners report using this pump on the following engines in place of late model Sherwood water pump:
• M-25XPB
• M-35B
• Some M-25XP
• Some M-35A
• Some M-320

The pump pictured is very similar to our part Z2880, but includes the extra machining steps that makes it a direct replacement. Should you want to consider modification of our Z2880 instead, please click this link to an article on the Catalina 34 National Association web site for extensive additional information.

Output Volume
• 9-1/2 gallons per minute at 2500 RPM and zero head
• 6-1/4 GPM at 1750 RPM and 10' head

• Pump housing
• Impeller
• Impeller o-ring
• Circular gasket that fits between the block and pump
1) Although the ports have the same threads as the the original Sherwood pump, 1/2" National Pipe Thread (NPT), the ports are slightly closer together than on the original Sherwood pump's ports. You must either to trim down one of your old elbows or replace it with our elbow, part Z3955, which is for the same pipe and hose size but is more compact than the original elbows. See "May we also suggest" below.

2) Universal’s "B" series engines are more powerful than earlier models. The M-35B has 20% more power than the M-35 and 56% more power than the M-25XP that it replaced. More power equals more waste heat. Late Sherwood pumps move more water than Oberdorfer pumps. That said, we have had many customers successfully retrofit Oberdorfer pumps even on "B" series engines without any issues. That makes it hard to know if the higher capacity is necessary or overkill.

If you have a smaller engine and sail in northern latitudes with colder water, you are likely to have more acceptable results with an Oberdorfer pump than if you are in warmer waters with a large engine. Keep a close eye on engine temperature after any retrofit to ensure that the new pump is up to the task. If yours is an M-35B engine, you have the most waste heat to reject and the greatest chance of overheating in adverse conditions.

Retrofitting an Oberdorfer pump to an engine that came standard with a Sherwood is always at your own risk. If you decide to try the retrofit and find the performance unacceptable, your only recourse is to purchase another Sherwood pump.
Once used, you may not return the Oberdorfer pump for performance reasons.

Technical Support

Water Pump Rebuild Tips
 Helpful tips for a task that requires careful work. Never run the pump dry!
Pipe Sizes and Why They Are So Confusing
Why doesn't my 3/8" NPT fitting measure 3/8"?