The Mercury Water Pump Impeller 47-89984T 4 combined with the Top Water Pump Housing Gasket 27-821304 is the core service kit for the raw-water pump in the Mercury Racing Super Speed Master #4 (SSM #4) gearcase.
This is the pump package that keeps race powerheads alive on boats running Mercury Racing S3000, SST-120, SST-200XS, S2000, USF1, F1H2O, and AUS F1 outboards. In the SSM #4, the impeller and gasket work together to pull water from the lake or river, seal the pump cavity, and feed a stable coolant supply to high-RPM two-stroke V-6 race engines.
Inside the SSM #4 pump housing, the 47-89984T4 impeller rides on the driveshaft in a dedicated race pump body, while the 27-821304 gasket seals the top of the housing against the gearcase and water tube interface. This combination maintains suction, discharge pressure and proper sealing, which are all critical on tunnel-boat and F1 engines that spend much of their life at or near wide-open throttle.
OEM Part Numbers and Technical Function
The 47-89984T 4 impeller is a Mercury / Quicksilver six-vane rubber impeller with a brass metal hub designed to survive high shaft speeds and repeated heat cycles. In a Super Speed Master #4 application, it is responsible for lifting water into the big vertical water tube and maintaining both flow volume and pressure to the powerhead and exhaust chest.
When the vanes age, take a permanent set or crack at the roots, the pump output drops and cylinder head temperatures begin to rise, often with very little warning.
The 27-821304 top water pump housing gasket is the primary static seal at the top of the pump assembly. It sits between the housing and gearcase, closing off bypass paths that would otherwise let water leak around, rather than through, the impeller chamber. A damaged or crushed gasket can cause aeration, reduced flow and erratic cooling even if the impeller is new.
Using the correct 27-821304 gasket with the 47-89984T4 impeller ensures that the pump operates as Mercury Racing intended, with a properly defined inlet, outlet and chamber volume.
Mercury Racing SSM #4 Applications
The Super Speed Master #4 gearcase is the classic small-diameter racing lower unit used under many Mercury Racing V-6 two-stroke engines. It is commonly found on S3000 Formula 1 powerheads, SST-120 and S2000 / SST-200XS tunnel-boat engines, as well as motors running in USF1, F1H2O, and AUS F1 championships.
When you check the Mercury Racing parts catalog for a Super Speed Master #4 gear housing, you will typically see 47-89984T4 listed as the water pump impeller and 27-821304 as the top housing gasket in the pump stack. If those numbers appear in your parts diagram, this impeller and gasket pairing is the correct OEM-spec service set for your SSM #4.
Service Interval and Installation Notes
On a race engine, the SSM #4 water pump is a consumable item. The impeller flexes with every revolution and is exposed to heat, sand, silt and aerated water, all of which accelerate wear compared to recreational use.
During service, the SSM #4 gearcase is dropped from the midsection, the pump housing is disassembled, and the old impeller and gasket are removed. The driveshaft, wear surfaces, cup and wear plate should be inspected for scoring or melting, and any heavily damaged components replaced.
The new impeller is installed with 2-4-C Grease, and the vanes are folded in the correct direction of the left-hand rotation as the housing is slid into place. The 27-821304 gasket is fitted cleanly to the top of the housing, ensuring full coverage of the sealing surface with no folds or debris trapped underneath.
Pump fasteners are tightened with a torque wrench to the specification in the Mercury Racing service manual for the specific engine and SSM #4 variant. The water tube must be properly seated in the outlet, and the gearcase should be pressure-tested before the engine goes back in the water.
A final water-pressure check on the hose, trailer or during a test run confirms that the new 47-89984T4 impeller and 27-821304 gasket are delivering stable cooling.
The Mercury Water Pump Impeller 47-89984T 4 and Top Water Pump Housing Gasket 27-821304 form the correct OEM water pump service kit for the Mercury Racing Super Speed Master #4 gearcase used on S3000, SST-120, SST-200XS, S2000, USF1, F1H2O and AUS F1 outboards.
Replacing these components on a regular schedule is one of the simplest and most effective ways to protect a high-value Mercury Racing V-6 powerhead, maintain consistent cooling, and keep an SSM #4-equipped tunnel boat running at full race pace from start to checkered flag.
Contact Mike Hill with any questions at +1-714-697-1716 or mike@buckshotracing77.com