Max Wawrzyniak - St Louis, Missouri - USA
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"
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
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
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
here for a list of Columns by Max Wawrzyniak