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replacing impeller raw water pump impeller pump impeller boat engine impeller

One of the most common items you'll replace on your boat is the water pump impeller.  Whether you are fixing a worn-out inboard engine water pump, rebuilding your stern drive or performing routine maintenance on your outboard motor, you'll likely be replacing an impeller as part of the job.

What is the Impeller, Exactly?

The impeller is a flexible rubber gear that creates the vacuum needed to pump water into your engine for cooling.  Since it is always in use when the engine is running, the impeller suffers a lot of wear and tear.  You should inspect the impeller at least once a year.  It is not uncommon for the impeller vanes to tear or break off, causing potential blockage in the cooling system and a loss of water pressure.  Additionally, impellers can swell, crack or bend, which can affect the ability of the pump to cool the engine.  Another possible defect is when the metal hub of the impeller detaches from the rubber, causing a situation where the hub is spinning but the rubber vanes are not turning and not pumping water.

If you observe that your impeller has sustained significant wear or damage, then you should replace it.  However, there are hundreds of impellers of various shapes, sizes and applications available on the market.  Knowing how to identify your impeller and identify its replacement can be challenging.  Many impellers may look similar, but have small, almost-imperceptible differences in dimensions. While these differences may seem inconsequential, they can actually result in big problems for your engine if you use the wrong one.

Know What You've Got

One of the most important steps you can take when replacing your impeller is to know the engine application.  Learn the brand of your engine, along with  year of manufacture and engine size. Also, keep the engine model and/or serial numbers handy.  These pieces of information can save a lot of research time and potentially avoid some catastrophic problems.  For Mercury, MerCruiser and Volvo Penta engines, the serial number is key.  For Johnson/Evinrude, Yamaha, Crusader, PCM, Indmar, OMC and other engines, model numbers are valuable for looking up parts.

If you are replacing the impeller in a bronze raw water pump, it may also be helpful to obtain the pump brand and part number.  Often, you can locate the part number of the pump on the pump body itself.  If the pump is older, this number may be worn or hard to read, but if you can get it, it will be a valuable piece of information to identify the right impeller for the application.

Many impellers may have a part number stamped on the side of the metal hub.  Jabsco and Sherwood impellers usually feature the part numbers in this way.  Many times, the impeller is too worn to read this number; it's also pretty small and hard to read.  However, if you can read it clearly, then you can quickly find the replacement without much research needed.

Doing the Research

Once you have the necessary information (part numbers, engine application, model/serial numbers, etc.), you can start searching for the correct impeller replacement.  There's a lot of good references online, but be careful!  Cross-reference information is not always correct, so verify that the impeller described matches the one you are replacing.  If possible, stick to resources published by the engine or pump manufacturer.  Also, double-check the features of the potential replacement compared to your original.  Do the number of vanes match?  Is the style of shaft the same (keyed, splined, flat sides, etc.). How about width and diameter?

By the way, if you are measuring width and diameter of an impeller that has seen a lot of action, be aware that your measurements may not match the factory specifications. After all, impellers can swell and deform during use, and using the physical dimensions to identify your replacement is not as accurate as using solid application information as described above.  We recommend using dimensions only as a secondary check once you have used your application information to identify what you believe is the correct impeller.

Get Everything You Need

Also, when replacing your impeller, don't forget to use new gaskets and o-rings!  Most impellers are available in kits that include all the necessary gaskets and o-rings to properly seal the pump.  We highly recommend you purchase an impeller kit instead of an individual impeller.  Do not re-use old gaskets and o-rings; even if they appear to be in good condition, they can have tiny imperfections that can cause big problems later.  And besides, gaskets are cheap - so do the job the right way!

Consult the Experts

Finally, if you have any questions finding the correct replacement impeller for your application, you can always rely on the knowledgeable people of Marine Parts Source.  Our staff has decades of experience identifying the right parts for dozens of different applications, and we've got lots of resource material at our disposal.  Get as much information as you can, then give us a call at 866-388-0390.  We'll help you identify the correct impeller for your engine so that you can get back on the water quickly.