You're due for a water pump service. The impeller alone costs $25-40. A complete kit runs $50-80. Is the kit worth the extra money, or are you paying for parts you don't need?
Here's when each option makes sense—and why most mechanics recommend the full kit.
Table of Contents
- What's in a Complete Kit
- What You Get with Impeller Only
- Price Comparison
- When to Buy the Full Kit
- When Impeller Only Makes Sense
- The Hidden Cost of Doing It Twice
- FAQ
What's in a Complete Water Pump Kit
A quality water pump rebuild kit typically includes:
| Component | Purpose | Typical Condition at Service |
|---|---|---|
| Impeller | Pumps water through engine | Worn vanes, cracked, or "set" |
| Wear plate | Provides seal surface for impeller | Grooved or scored |
| Housing gasket | Seals pump housing to lower unit | Compressed, may leak |
| O-rings (2-3) | Seal water passages | Hardened, may leak |
| Impeller key | Locks impeller to driveshaft | Worn or damaged |
| Water tube seal | Seals tube to powerhead | Often overlooked, often leaking |
Brand-specific notes:
- Yamaha kits usually include 2-3 O-rings plus the water tube grommet
- Mercury kits often include an insert cartridge on newer models
- Johnson/Evinrude kits may include the housing on some models
What You Get with Impeller Only
Just the impeller. That's it.
You're reusing: - The old wear plate (which has been grinding against rubber for 100+ hours) - The old gaskets (which have been heat-cycled and compressed) - The old O-rings (which have hardened over time) - The old key (which may be worn)
If any of these fail after you reassemble, you're pulling the lower unit again.
Price Comparison
Real pricing for a typical 75-115 HP outboard:
| Option | Cost | What's Included |
|---|---|---|
| Impeller only | $25-40 | Impeller |
| Complete kit | $50-80 | Impeller + wear plate + gaskets + O-rings + key |
| Complete kit + housing | $100-150 | Everything above + pump housing |
The math: - Kit premium over impeller: $25-40 extra - Cost to pull lower unit again: 1-2 hours labor ($100-200 at a shop) - Cost of overheated engine: $1,000-5,000+
When to Buy the Complete Kit
Always buy the full kit if:
- It's been more than 2 years since last service
- Gaskets and O-rings degrade with time, not just use
-
Rubber hardens even sitting in storage
-
You're doing the job yourself
- You're already investing time in the work
- Parts cost is a small fraction of your time
-
One bad O-ring means doing it all over
-
You found rubber debris in the old impeller area
- Pieces went somewhere—probably into passages
-
Full kit lets you inspect and clean everything
-
The engine has been overheating
- Heat damages more than just the impeller
-
Wear plate warps, gaskets fail, seals harden
-
You're buying aftermarket anyway
- Quality aftermarket kits from suppliers like SeaSierra cost about the same as OEM impeller alone
- No reason not to get everything
When Impeller Only Makes Sense
Consider impeller-only if:
- You just did a full kit last year
- Running in sandy water and need mid-season replacement
-
Gaskets and seals are still fresh
-
You're a shop doing high-volume work
- You inspect each component individually
- You stock gaskets and O-rings separately
-
You can assess wear plate condition accurately
-
Emergency repair situation
- Impeller failed mid-season
- Kit not available, impeller is
- Plan to do full kit at end of season
Reality check: Even in these cases, most experienced mechanics still use the full kit. The parts are cheap, and callbacks are expensive.
The Hidden Cost of Doing It Twice
Here's what happens when you cheap out on the kit:
Scenario 1: Wear plate is grooved - Old wear plate has a 0.5mm groove from the impeller - New impeller can't seal properly against the groove - Reduced water flow, engine runs hotter than it should - You don't notice until midsummer when temps spike
Scenario 2: O-ring fails - You reused the housing O-ring because "it looked fine" - Three weeks later, small leak develops - Water flow drops 10-15% - Tell-tale looks normal, but engine runs warm
Scenario 3: Gasket weeps - Old gasket was compressed to half its original thickness - It seals... mostly - Small amount of water bypasses the pump - Hard to diagnose, easy to prevent
In all three cases: You're pulling the lower unit again. That's another hour of your time minimum, plus the parts you should have bought the first time.
What About the Housing?
The pump housing itself isn't usually included in standard kits. You need it if:
- Housing is scored (you can catch a fingernail in the grooves)
- Housing is cracked
- Previous impeller failure sent debris into the housing
Check your housing when you have it apart. Run your finger around the inside. If it's smooth, reuse it. If it's grooved, replace it. A scored housing will eat your new impeller in one season.
Housing replacement adds $40-80 to the job, depending on the engine.
OEM Kit vs Aftermarket Kit
OEM kits from Yamaha or Mercury run $70-120. They fit perfectly and include everything.
Generic aftermarket kits vary wildly. Some are fine, some aren't. The impeller rubber compound matters—cheap rubber hardens fast and pumps poorly when cold.
Quality aftermarket from suppliers who source from OEM factories—like SeaSierra—gives you the same materials and tolerances at 30-40% less. For a wear item you're replacing annually, that's real savings without the risk.
FAQ
Can I reuse the wear plate if it looks okay?
Run your fingernail across it. If you feel any grooves, replace it. "Looks okay" isn't good enough—even shallow grooves affect the seal between impeller and plate.
How do I know if the O-rings are still good?
You don't, really. They may look fine but have hardened internally. At $2-3 per O-ring, it's not worth the risk.
What if my kit doesn't include the water tube seal?
Some kits don't. Check what's included before you order. If it's not in the kit, order it separately—it's a common leak point and a pain to access later.
Should I replace the impeller key every time?
Yes. It's a $2 part that keeps the impeller locked to the shaft. A worn key lets the impeller slip, reducing flow. No reason to reuse it.
Is it worth buying OEM?
OEM fits perfectly and works well. But quality aftermarket from reputable suppliers uses the same materials at lower cost. For consumable parts like water pump components, aftermarket makes sense—just avoid no-name imports.
Bottom Line
The complete kit costs $25-40 more than the impeller alone. That's the price of one hour at a shop—or half a tank of gas.
If any of those cheap parts fail, you're doing the whole job again. If the engine overheats because of a bad seal, you're looking at serious money.
Buy the kit. Replace everything. Do it once.
Find water pump kits for your engine at SeaSierra.
