Whale AK8050 Service Kit | Henderson MK5 Bilge Pump

The Whale AK8050 is the complete service kit for the Henderson MK5 universal bilge pump, the one Whale sells under part number BP0510. If your manual bilge pump has gone soft on the stroke, started leaking around the cover, or just isn't moving the water it used to, this is the fix. You rebuild the wet end instead of buying a whole new pump. For a pump that's often bolted into a locker or bonded to the hull, that's the difference between an afternoon job and a much bigger one.

Everything that wears out is in the box. You get a fresh diaphragm, a new inlet valve and inlet valve plate, a new outlet valve, and the top cover seal that keeps the whole thing from weeping. Whale includes the small parts that always seem to walk off during a rebuild too: two large circlips, two small circlips, four long valve screws, and a full set of eight M5 by 25 screws with eight matching M5 nuts. So you're not hunting through a parts drawer halfway through the job.

The rebuild itself is straightforward if you take your time. Pull the top cover, lift out the old diaphragm, and swap the inlet and outlet valves. The diaphragm is the heart of the pump, and a cracked or stiff one is usually why the pump lost its prime. Get the new valves seated flat and aligned before you close it up. We've seen diaphragms tear again within a couple of seasons when the valves weren't lined up right, so it's worth slowing down here. A light smear of waterproof grease on the seals helps them seat and makes the next rebuild easier.

Two things trip people up. The circlips are the first. They want to pop off on the first stroke if they aren't fully seated, so use a proper pair of circlip pliers and steady, even pressure until each one clicks home. Keep the spare small circlips close, because they're easy to launch across the bilge. The second is the top cover seal. That's the most common leak point after a rebuild, and it's almost always because the cover nuts got torqued unevenly and warped the cover. Snug the eight M5 nuts in a cross pattern, a little at a time, the same way you'd torque a wheel.

The kit also gives you fresh mounting hardware, which matters more than it sounds. M5 screws strip easily when they're run straight into a fiberglass mount, so pre-tap the holes and add a dab of thread locker on reassembly. Do that once and the pump stays put through years of pounding.

This kit fits the Henderson MK5 on both powerboats and sailboats, and it's a smart part of an annual maintenance plan if your bilge sees mud, silt, or oily water. Those bilges gum up the valve plates faster, and a yearly clean-and-rebuild keeps the pump ready for the day you actually need it. If you're already in there, it's worth checking the outlet hose and check valve at the same time, especially on a sailboat that spends time heeled over, where backflow through a tired valve is a real problem.

One note on fitment: this is the service kit for the MK5 version of the BP0510 pump. If you're not sure which generation of Whale Henderson pump you've got, check the casting on the body before you order, or send us the numbers and we'll confirm it. Get the right kit and the rebuild is genuinely satisfying. Bad pump in, good pump out, and you didn't have to unbolt the whole thing.