It should be a little bit over 1/2". I think the measurement is . 550".
The overflow valve is on the engine side of the injection pump even with the first delivery valve. Before you take it off, cram some rags between the pump and engine below the valve to catch the washers. The valve looks like a banjo fitting. Don't take the small hex head screw out until you remove the valve and put it on your work bench. You can get to the spring when you remove that screw.