Water Pump Identification
By Pass vs. NON By Pass
Flat Head 6 Cylinder
By Neil E. Wojciechowski
I have been asked what is the difference
between a by-pass and a NON by-pass pump on the Chrysler flat
head six engines. First ALL pre-1951 Chrysler FLH engines were
of the NON- by-pass design. They used what is called an EXTERNAL
by-pass to keep water moving through the engine until the thermostat
opened up and let the water circulate through the radiator. In
1951 a design change was made to the engine block which allowed
for the use of an INTERNAL by-pass water pump. I have taken PICs
of the two different pumps and the backing plates to help in identification
of the differences. One basic rule of thumb in switching between
the two different types is you can use an INTERNAL by-pass type
water pump on an EXTERNAL type engine, but you CANNOT use a NON
by-pass (or EXTERNAL by-pass) pump on an INTERNAL by-pass type
engine. A NON by-pass type on an INTERNAL engine will cause the
engine to spot over heat causing exhaust valve seat damage.
This is an example of a NON by-pass (EXTERNAL
by-pass) pump. Circled in red shows that there is no hole. Compare
it to the next PIC.
This is an example of a INTERNAL by-pass pump.
Circled in red shows the INTERNAL by-pass hole. Compare it to
the previous PIC.
This is a PIC of the two different backing
plates used on the pumps.
You can use the NON by-pass pumps backing
plate on a by-pass type to eliminate the internal by-pass. This
is recommended if you replace a NON bypass pump with a by-pass
type and you develop a leak between the block and the pump. I
have never had that happen, but it is described that way in repair
manuals.
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