Mercury – Complete Water Pump Kit | 9.9–20hp EFI Series | Marine-Grade Components | 8M0135821

Overview

There’s a certain confidence that comes from knowing your outboard’s cooling system is truly up to the job. When you’re running through a narrow channel on a rising tide or easing into a long stretch of open water, you want to trust that your engine is staying cool, steady, and protected. The Mercury Marine part 8M0135821 Complete Water Pump Kit gives you that peace of mind. It’s the kind of reliability that lets you focus on the moment — the sound of the hull gliding, the breeze on your face — instead of whether your outboard is running too warm.

Key Features

• Complete water pump kit for Mercury 9.9–20hp EFI outboards.
• Fits Mercury 9.9hp EFI ProKicker 333cc (serial 1R008146–1R278792).
• Fits 10–20hp 2-cylinder EFI and SeaPro outboards (serial 0R833820 and up).
• Includes water pump case, two gaskets, oil seal, liner, impeller, guide plate, key, and seal.
• High-flexibility impeller elastomers designed for broad seasonal temperature ranges.
• Marine-grade construction for long-term durability.

Engine & Boat Compatibility

• Mercury 9.9hp EFI ProKicker (333cc).
• Mercury 10–20hp EFI FourStroke.
• Mercury SeaPro 10–20hp models.
• Perfect for small skiffs, lightweight fishing boats, tenders, utility craft, jon boats, and inflatables.

Performance & Practical Benefits

A water pump is one of those parts you don’t think about until it isn’t doing its job. This kit is designed so you never have to think about it. With every turn of the impeller, the engine gets the cooling flow it needs — steady, reliable, and predictable. You’ll notice smoother running at idle, less heat-related strain during heavy loads, and a general sense of confidence when you push the throttle.

The impeller is built with elastomers that stay flexible across seasonal swings, whether you’re running in cold spring water or the warm shallows of midsummer. The fitment feels tight, precise, and clean during installation, which makes a big difference when you’re servicing the pump yourself. Everything in the kit has its place, and once installed, it creates a sealed, efficient cooling path that protects your engine from the inside out.

Long days on the water demand a cooling system that can keep up with the engine’s rhythm. This kit does exactly that — it stays steady, resists wear, and keeps coolant flow consistent, even after hours of running.

Why Buy OEM

When it comes to cooling systems, guessing isn’t worth the risk. Mercury OEM parts are engineered around exact tolerances, materials, and flow rates designed specifically for their EFI outboards. Aftermarket kits might look similar, but they often lack the consistency and durability needed in real marine conditions. With a Mercury Marine part, you’re getting something you can install once and trust for seasons ahead. It’s a smarter investment, a safer choice, and exactly what most experienced boaters recommend.

FAQ

Q1: Which Mercury engines does the 8M0135821 kit fit?

It fits Mercury 9.9hp EFI ProKicker engines and 10–20hp EFI and SeaPro models with the matching serial ranges.

Q2: How is this different from aftermarket water pump kits?

OEM kits use the exact elastomer compounds, sealing surfaces, and dimensions Mercury engineered for proper cooling and long service life.

Q3: How do I confirm this is the right kit?

Check your serial number. If your outboard matches the listed ranges, this is the correct replacement.

Q4: Is this suitable for both saltwater and freshwater?

Yes, the materials are designed for tough marine environments and resist corrosion and wear in both conditions.

Q5: Is installation manageable for most boat owners?

If you’re comfortable servicing your lower unit, this installs cleanly. The included parts make the job straightforward.