Milky, gray gear lube in the lower unit means water intrusion. Once the oil emulsifies, film strength collapses, and gears wear fast. A frequent and easily missed leak path is at the water pump base: the driveshaft oil seal and the base O-ring live here. If either hardens, nicks, or the base bore is worn, water can enter under load. Replacing the water pump base assembly restores sealing right where the driveshaft exits the gearcase.
 
This kit includes a new base, the primary seal, and the correct O-ring, making it the proper fix when you suspect leakage around the shaft or the base rather than the impeller or housing. It’s also smart to replace it whenever the gearcase is down for cooling-system service and you notice groove wear in the base or damage to the O-ring land.
 
Fitment covers many Mercury and Mariner V6 two-stroke platforms, including 2.0L, 2.4L, and 2.5L families: V-135, V-150, V-175, V-200, V-220/Laser, XR2, XR4, XR6, XRi, Pro Max 225, 2.5 Sportmasters, 2.4 CLEs, various Magnum and EFI/DFI serial ranges, plus 105/110/140 Jet applications. Later 2.5L Pro XS variants also appear in where-used lists. Always confirm by engine serial number, as mid-run changes and gearcase variants exist.
 
From a parts standpoint, Mercury base assembly 46-42579A3 (often shortened to 42579) is superseded by 8M0077147, which is the current OEM service number. Common cross-references used in catalogs include Sierra 18-3419, GLM 12360, and Mallory 9-43307. For sealing integrity on suspect gearcases, many technicians stay with the OEM base and seal stack.
 
Installation is straightforward.
 
Drop the gearcase per the service manual and support it on blocks to protect the skeg. Remove the pump housing and wear plate to expose the base.
 
Extract the old base without scoring the register, then clean all mating surfaces; corrosion bloom or gasket imprint beneath the O-ring land can telegraph into a new leak.
 
Lightly oil the new seal lip and verify the garter spring is seated.
 
Fit our pre-assembled base with the O-ring squarely in the groove, then guide the base over the driveshaft and onto the gearcase boss with even pressure so the lip doesn’t roll.
 
Reinstall the wear plate and housing, align the impeller key, and torque fasteners in a cross pattern to spec. Before filling with fresh High Performance Gear Lube, perform a pressure/vacuum test; a stable hold confirms the repair.
 
Finish with a brief run on the hose and recheck for drips.
 
If your lower-unit gearcase oil is already milky, don’t run the engine again until you’ve corrected the leak and replaced the lube. When prop-shaft and shift-shaft seals test tight, the water pump base—specifically the driveshaft seal and base O-ring—is the prime suspect. Replacing the base, seal, and O-ring as a set is inexpensive insurance compared to gears and bearings.
 
Part numbers: 46-42579A3, 42579, 8M0077147, 12360, 18-3419, 42579A3, 509412, 9-43307
 
Select: 
1) Standard - for most applications 
2) Older- for Push-Pin Drivershaft Lower Units
 
Contact Mike Hill with any tech questions at +1-714-697-1716 or mike@buckshotracing77.com