Two Early Brass Drain Petcocks for Radiator & Engine Block.

Are they 1/8 inch or 1/4 inch pipe thread?

Hi, Mike. I'm glad you asked. I previously checked the sizing for another guy, and meant to post that information here.

The treaded area of the petcock is 1/8" npt (national pipe thread), which is surprisingly large in reality. Roughly, the outer diameter of this treaded area measures about 7/16" or a hair over .40". The overall length measurement from where the petcock would start to screw in to the hole in the engine block to where the water or antifreeze would exit is 1-3/4". It is 1-3/8" from the base of the threads to where the water or antifreeze would exit. The distance from the top of the thumb screw on the valve to the bottom of the housing measures 1-3/8".

Just let me know, if you have any other questions. Thanks. John