How to Rebuild an Outboard Motor Water Pump

If you read my post 15 Outboard Motor Maintenance Blind Spots You Can’t Afford to Miss two weeks ago and discovered that the water pump (#7) of your outboard motor is one of your maintenance blind spots, then this post is for you. It’s also for you if you didn’t read that post and you don’t have a clue how to maintain a water pump or if Google sent you here while you were looking for how to replace an impeller.

In that earlier post, I encouraged you to flush your cooling system often to prevent a clogging and to reduce wear to your water pump. I showed you the inside of a water pump and explained how the impeller can wear or break. And I promised to show you how to replace the impeller in a future post. This is that post but I’m going to do you one better and show you how to rebuild the pump entirely, not just replace the impeller. If you only want to replace the impeller, you can also use this post for that, just ignore the bits about replacing the other parts.

Most articles and videos on the web about impellers only cover how to replace the impeller and ignore the rest of the water pump. When I was shopping for an impeller for my outboard (a 2004 Yamaha 8 HP four-stroke with long shaft), I had the choice of buying just the impeller or the entire service kit. I opted for the kit since this was the first time that I had opened up this particular water pump and I’m glad I did. The impeller had probably been replaced before but there was other wear inside the pump that had not yet been repaired.

Water Pump Anatomy 101

The pictures below show it easier than I can explain it. While these pictures are from my outboard motor, to my knowledge, most other makes have a very similar design, so they should apply to your outboard too.

The first picture is of what’s called the outer plate. It’s the bottom of the water pump. The drive shaft goes through it and the impeller sits flat on top of it. The old plate is on the left, a new plate is on the right. You can plainly see the grooves worn in the old plate from the impeller. They mean a weaker seal and potentially faster wear.

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Old outer plate (L) and new outer plate (R)

These three pictures also make it easy to see how an outboard motor water pump works. Notice in the picture above how the shaft of the pump isn’t in the center of the pump body but is offset a little toward the top of the pump cavity. That isn’t an optical illusion, that’s how it’s designed. Imagine the star-shaped impeller (below) rotating clockwise around the center hole. The impeller pulls water into the pump through the semi-circular slot. When a blade passes the top of the slot (about the 2 o’clock position), it’s like a valve opening and admitting water. As the impeller rotates, the spaces between the blades on the right side of the pump get larger. That causes suction that pulls water into the pump. As a blade passes the bottom of the slot (about 5:30), the valve closes, trapping the water between the blades while the spaces between the blades are the biggest.

As the impeller continues to turn up the left side of the pump, the water trapped between the blades gets compressed as the spaces between the blades get smaller and smaller. The impeller blades bend to still fit in the smaller space.

That’s why the blades on one side of the old impeller are bent in the picture below and the blades on the other side are straighter. The bent blades were in the smaller side of the pump body when I removed the impeller. When they are new, all the blades are straight like the new impeller on the right in the picture below.

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Old impeller (L) and new impeller (R)

When the blades get to about the 10 o’clock position, they pass into another semi-circular slot, in the top of the pump body this time (see the picture below). The compressed water shoots out the top of the pump through a tube to the engine where it circulates before squirting out the back of the motor. The water pump works similar to a rotary combustion engine (think Mazda RX series sports cars).

In the picture below, you can see scoring in the top of the pump body similar to the bottom plate. Also notice the corrosion stain in the center. That’s from salt water seeping down through the top of the pump and sitting on top of the impeller, trapped by the impeller seal.

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New cartridge insert (L) and old cartridge insert (R)

Now that you understand better how a water pump works, the video below will make more sense as you watch me disassemble the pump and then reassemble it with new parts. Please forgive the focus problems. The autofocus on my camera went spastic a couple of times.

Casualty of Wear

In the middle of the video, you see where one of the water pump bolts snapped off due to galvanic corrosion. What I left out of the video was me removing the broken bolt. It turned out surprisingly well, actually. I was fortunate that the bolt broke about 1/2″ above the lower unit casing and not flush or below the surface. After I carefully pried the top plate off the casing, I soaked a strip of paper towel in ordinary vinegar and wrapped it around the exposed part of the bolt so that the vinegar could seep into the threads over time.

I kept it saturated and after a few days, I was able to lock two nuts together on the exposed end of the bolt and back it out easily. The vinegar did a great job of dissolving the aluminum oxide. The same trick probably would have worked even if the bolt had snapped off flush, I just would have had to drill into the bolt far enough to grab it with a bolt extractor instead. The next time you discover some galvanic corrosion, unless you’re a gambling man (or woman), reach for the vinegar before things break.

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Three and one half bolts came out without problems

I’ll be adding that bolt to my growing Hall of Shame in the post titled Beware of galvanic corrosion. So far, galvanic corrosion has cost me around $100 in replacement parts that broke while trying to remove seized fasteners. Even more costly is the time lost from sailing, waiting for parts, and replacing them. I had to wait 15 weeks for a replacement masthead alone. Thank goodness a friend loaned me his spare masthead in the mean time. If the previous owner of Summer Dance would have paid better attention to corrosion prevention, it would have cost him far less than that and would have cost me nothing. Don’t neglect your own boat!

Before I close this post, I want to recommend a resource to you for outboard motor parts diagrams and replacement parts, Boats.net. I can easily find exactly what I need there and get the parts quickly at a fair price. They’re dealers for Evinrude, Honda, Mercury, Suzuki, Tohatsu, and Yamaha. Perfect for any stingy sailor. I’m not affiliated with them in any way, I just like their site.

There you have it – how to get the parts you need and how to put them in. I hope this post helped you to understand how your outboard motor’s water pump works and how easy it is to maintain. If you’re up for it, don’t spend potentially hundreds of dollars for someone else to do it for you, do it yourself! Then just keep an eye on the rate of wear each year and it won’t be a maintenance blind spot.

The bottom line

Suggested price: $150-$300
$tingy Sailor cost: $41 ($25 for impeller only)
Savings: $109-$259


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2 Comments Add yours

  1. capnrehab says:

    Thanks for making a video for this. It made it much more clear.

  2. Peter says:

    Thanks so much ! Nice tutorial.

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