Honda BF8D-BF20D Water Pump Impeller Replacement

Introduction

This guide will help people of any experience level through the steps for repairing the impeller in the water pump. In a boat’s engine, the water pump is the engine’s version of a transmission, so this part is essential. With a broken impeller, the engine will overheat since water isn’t being pumped through the core. With a few tools and some spare time, this quick and easy fix can be done at home, which will save money in the long run. Now, most of the bolts are 10mm so having 10mm tools will be a must. Also, an impeller replacement kit is needed to swap out the broken impeller. This is a very safe fix, but wearing surgical gloves is optional for sanitary reasons.

What you need

Step 1 Impeller

Honda BF8D-BF20D Water Pump Impeller Replacement, Impeller: step 1, image 1 of 2

Honda BF8D-BF20D Water Pump Impeller Replacement, Impeller: step 1, image 2 of 2

Shift the engine into reverse.

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Step 2

Optional: Remove the cotter pin from the propeller hub with pliers, then remove the bolt holding the propeller on. After that, remove the propeller.

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Step 3

Use a 10mm wrench to loosen the shift shaft connection located at the front of the extension case. It can be seen in the access hole.

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Step 4

Honda BF8D-BF20D Water Pump Impeller Replacement: step 4, image 3 of 3

Remove the five 10mm bolts holding the lower unit to the extension case using either a wrench or ratchet.

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Step 5

Gently tap the lower unit/gear case with a rubber mallet until it becomes loose.

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Step 6

Carefully lower the lower unit/gear case down and remove it from the rest of the engine.

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Step 7

Honda BF8D-BF20D Water Pump Impeller Replacement: step 7, image 3 of 3

Remove the four 10mm bolts with 10mm socket wrench.

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Step 8

Honda BF8D-BF20D Water Pump Impeller Replacement: step 8, image 3 of 3

Lift the water pump/impeller housing off the drive shaft.
Remove the water pump impeller. Try to remember its exact position as that will be important during a later step.
Pay careful attention to the woodruff key holding the impeller on the shaft. You may need this later if your replacement kit does not come with one. The key has been marked in yellow so you know what part it is as it is small.

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Step 9

Remove the bottom metal impeller housing plate.
Scrape the old gasket off.
Optional but recommended: With everything disassembled, inspect and clean corrosion/sand out of the lower unit/gearcase.

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Step 10

Honda BF8D-BF20D Water Pump Impeller Replacement: step 10, image 3 of 3

Open the replacement water pump/impeller kit.
Replace the base plate gasket.
Replace impeller housing O-ring.

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Step 11

Install the water pump/impeller into the housing in correct direction by pushing and rotating it so that all impeller blades face the same correct direction. All blades should be slightly bent to the right in the housing.
The woodruff key slot should be noted. This has been marked in orange to clarify the location.

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Step 12

Hold the woodruff key in place with a pair of pliers. Slide the water pump/impeller housing down the shaft and align the woodruff key slot on the impeller with the woodruff key. Align the slot and key, then rotate it into place.
If this proves to be difficult, remove the metal impeller race from the plastic impeller housing and try again.

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Step 13

Reinstall the 4 impeller housing bolts.
Grease the driveshaft and especially the splined section of the driveshaft with marine grade grease.
Reinstall the lower unit/gearcase into the engine. This may require you to rotate the shaft to allign the splines.
Reinstall the 5 10mm bolts that hold the lower unit/gearcase and tighten to 9 Foot-pounds of pressure. These bolts were the ones removed in step 4.
Reconnect the shift linkage.