Posted by Neil on Wednesday, May 30, 2001 at 2:50PM :
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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