Can anyone tell me the proper hole size for the bypass in a #326 water pump? I see many of the after market suppliers stating 3/4" is not correct. So was the 63 outlet size smaller? Thanks Larry
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Re: 63 Water Pump Bypass
Larry-----
The original '326' water pumps used on the various pre-1968 small blocks which used external bypass, were of 5/8" NPT thread size. Later service replacement pumps or '326' castings originally used for other applications use the 3/4" NPT thread size. Adapters are available to convert them but, I understand that "eagle-eyed judges" are known to scrutinize carefully for evidence of this feature and penalize the infidels that so-modify "incorrect" waterpump castings.In Appreciation of John Hinckley -
Clarification needed
I don't have the original casting (replaced in '68), but there is a reducer bushing between the pump housing and the hose nipple fitting. Is this the OEM configuration?
DukeComment
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Re: 63 Water Pump Bypass
Always discussion(s) on this 1/2 vs 5/8 NPT or 5/8 vs. 3/4. Big picture is given by others.... Correct Corvette application has SMALL hole.
Reason for confusion here is distinction between casting number (326) and actual GM part number (not shown on pump). Casting was what came out of foundry. After ancillary operations (drill & tap upper boss, install impeller, shaft, bearing/seal, and fan drive) finished item took on a part number.
Most trucks of the era used a different P/N from the same casting. This featured the bigger bypass outlet. Many water pump rebuilders had a common brainstorm -- take all 326 castings, 'punch' them to the largest opening and toss an NPT-NPT adaptor into the box to create a 'universal' water pump. Hence, finding an original '326' with small opening is hard since so many have been through one or more rebuild cycles....Comment
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Re:5/8 for sure
Jack. I have in hand a GM pump with the full part number cast on the front with a 5/8 inch outlet hole. Is it your intention to suggest this is not a Corvette pump? LarryComment
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Re:5/8 for sure
Larry----
Jack was correct in his post and I was off the mark in my earlier post. Corvettes with the '326' water pump did use a 1/2" NPT fitting size on the upper bypass. In fact, that's what I should have said, but I was operating entirely from memory and that fails from time-to-time; I just couldn't remember whether the Corvette fitting was 1/2" NPT or 5/8" NPT. I am not even sure that any '326' castings were drilled for 5/8" NPT; bear in mind that 1/2"NPT is about 3/4" in diameter. 3/4" NPT is about 1" in diameter.
If you actually have a pump with 5/8" NPT by-pass fitting, then perhaps some were made or someone enlarged a 1/2" NPT to 5/8"NPT. Incidentally, 5/8" NPT will measure about 7/8" in diameter.
MANY '326" pump castings were produced with 3/4" NPT by-pass fittings. In fact, since about 1970 or so, most small block GM SERVICE water pumps for short leg, pre-1971, small block water pump applications were '326' castings with 3/4" NPT by-pass fitting. And you can bet that they made more than a few of those.In Appreciation of John HinckleyComment
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