by Max Wawrzyniak - St Louis, Missouri - USA   We're Gonna Pump You UP!

One of the most commonly-replaced parts on an outboard motor is the water pump impeller. Some folks consider this a good reason to buy air-cooled outboard motors, but some supposidely air-cooled outboards still utilize a pump to cool the exhaust housing so you may not avoid the pump with an air-cooled outboard.

For the old outboards I always recommend (OMC-manufactured outboards from about 1955 to the early 1970's) replacement pump impellers are easy to obtain, both from Johnson and Evinrude dealers and also from after-market suppliers such as Sierra. After-market impellers can be purchased from any boat dealer (not just those delaers that carry the Johnson/Evinrude brands) and also through NAPA auto parts stores.

Some of the large marine discount stores also handle a few impellers.

For other brands of old outboards, the replacement pump impeller situation is a bit cloudier. Although impellers for most old Mercury outboards can be found, the are usually a bit more expensinve than OMC (Johnson/Evinrude/Gale) impellers, probably because the volume of Mercury parts sold is less.

Replacement impellers for more obscure outboards, such as Scott-Attwaters, Champions, and Olivers can be much harder to find. When such impellers can be found, they are usually very expensive low-volume production items, or they are decades-old parts that someone found on a dusty shelve somewhere. These NOS ("New-old stock") "left-over" parts may or may not have aged gracefully, and I have heard of decades-old "new" impellers that disintigrate after relatively few hours of running.

One of the most commonly-replaced parts on an outboard motor is the water pump impeller.

The question that the owner of an obscure outboard usually asks is if there is an existing impeller that will fit his/her outboard? There have been literally hundreds of impellers manufactured for outboard motors over the decades since World War II so there is a fair chance that there is something out there that will fit, or will fit with only minor modifications.

The problem, of course, is how does one find this "will-fit" impeller? Most parts distributors are loath to allow one to poke around in their stock shelves, and even fewer are willing to poke around in the back room themselves trying to "match-up" an impeller.

About a decade ago, an intrepid member of the Antique Outboard Motor Club took the initiative to tackle this situation. Somehow (I don't recall how) he was able to find the dimensions and physical charachteristics of the water pump impellers manufactured by Sierra, which makes after-market replcement impellers for many brands of outboard motors as well as some inboard engines and sterndrives. This gentleman then complied a list of these details which he then mailed out to many members of the Antique Outboard Motor Club.

Although I remember that he specifically intended for this list of impellers to be distributed freely among the members of the club, I have never seen this list published anywhere. So I am publishing it here.

Keep in mind that mistakes can be made, so there is certainly no gaurantee that the information on the list is error free. Also, as this list is about a decade old, it is possible that some of the impellers listed may no longer be in production.

The details listed are over-all diameter, height (thickness), bore diameter; whether the impeller is "locked" to the driveshaft with a "key" or with spines, and the number of blades ("Paddles.")

I have seen someone reduce the thickness of an impeller by sanding it on a belt sander, and the impeller worked, at least initially. I don't know if he kept the motor long enough to know if the impeller worked long-term. An impeller that is too thick will overheat and literally melt, while an impeller that is not quite thick enough will not pump water.

Some impellers have (2) keyways (Notches)180 degrees apart. These should be OK on an engine with a single key, all other factors being equal. Conversly, an impeller with a single keyway might be adaptable to having a second keyway cut or filed into it if the metal or plastic bushing is thick-enough to allow it.

Keep in mind that in most cases, the system that "locks" the impeller to the driveshaft allows the impeller to move up and down on the driveshaft a slight amount, so that the impeller is not forced against the top or the bottom of the pump housing. One can not just rigidly pin or otherwise fasten the impeller to the driveshaft.

An impeller with a thick plastic bushing at it's center might be adaptable to having the bore drilled to a larger diameter on a drill press. An impeller with a metal bushing probably does not have enough "meat" in the bushing to allow it to be drilled to a larger diameter. Or an impeller with a large bore might be fitted with a bushing to reduce the bore diameter. Maintaining the centering of the bore is vital in all the above alterations.

Beyond that, I am afraid that you are on your own. The availability of such parts is a big part of the reason that I recommend the old OMC outboards, but if you already have an old outboard of a different brand, you might find a "will fit" impeller with the aid of this list.

My thanks to the originator of this list, whose name and address I have removed from it in order to preserve his privacy.

Happy Motor'n
Max

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