C2 Water Pump bypass fitting size

[Alan Drake]

I'm back to the water pump on my Mar64 car. Saved the right angle bypass fitting from top of water pump. However not sure if its the right size. Know (dangerous word to use when talking about corvettes) it should be 1/2" NPT not the latter 3/4" NPT - my problem is lack of knowledge of NPT sizes.

The part measures 1/2" for an inside dia while the outside of the threads measure .78" and then increase to .85". The part has the correct shape an has the 4 diamonds going up one side with a "P" in the center of each.

So before I go off and get a 3839175 pump, need to known if a fitting is also required???

If the fitting is incorrect does the 1/2"NPT have the same size outlet to manifold as the one I have?? Again it measures 1/2" inside dia with a lip of .875" which matches up to the FI intake manifold.
Thanks

[Joe L.]

Re: C2 Water Pump bypass fitting size

Alan-----

Your fitting sounds like it's the correct 1/2" NPT. A 1/2" NPT fitting does not measure 1/2" OD. It measures about the size you've described.

A correctly drilled and tapped GM #3839175 pump casting will use the exact same 1/2" NPT fitting that you have. However, beware of castings that have been drilled and tapped for the next oversize fitting---3/4" NPT. Sometimes, rebuilders or others do this to castings originally drilled and tapped for 1/2" NPT. Also, some SERVICE waterpumps originally using the 3839175 casting may have been drilled and tapped for 3/4" NPT.