The water pump base O-ring, listed under Mercury Racing part numbers 25-39685 and 39695, is a vital sealing component used in high-performance outboard engines built for the most demanding racing applications.
This O-ring is specifically designed for outboards fitted with the Super Speed Master #4 (SSM #4) gearcase, a surface-piercing lower unit relied upon in professional powerboat racing environments.
Engines such as the Mercury Racing S3000, SST 120, S2000, and the current-generation APX outboards—including the 200R APX, 250R APX, and 360R APX—depend on a consistent and sealed water flow system to maintain proper engine temperatures under high-load conditions.
The water pump base O-ring is seated at the interface between the gearcase and the water pump housing, creating a critical seal that directs water into the engine cooling passages.
In competitive formats like F1H2O World Championship, US F1, F200, and ChampBoat racing, where Mercury Racing outboards are pushed to their performance limits, the efficiency of the cooling system is non-negotiable.
This small rubber O-ring ensures that no water escapes and that pressure remains steady as water circulates through the engine, particularly important when engines are operating at sustained high RPM in closed-course race environments.
As durable as it is, the 25-39685 / 39695 O-ring is still subject to degradation over time. Heat, pressure cycles, and exposure to water and lubricants can cause the rubber to harden, crack, or compress.
A compromised seal can lead to a drop in water pressure and cause water intrusion into the gear case, any of which may cause engine overheating or damage to the gears, especially in racing scenarios.
For this reason, replacement of the water pump O-ring should be checked every time the SSM #4 gearcase is removed for maintenance.
In conclusion, whether you're fielding a boat in the F1H2O series, competing in a US F1 Grand Prix, dialing in a rig for F200 class racing, or pushing limits on the ChampBoat circuit, the integrity of your SSM #4 lower unit water pump system is critical.
The Mercury Racing water pump base O-ring, part number 25-39685 or 39695, is a small yet essential component that supports optimal cooling system function and overall engine longevity.
Proper maintenance and timely replacement of this O-ring help ensure your racing outboard performs with maximum reliability, power, and consistency—every lap, every race.
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.