Oil Pressure Gauge - Plastic Tube Removal

The technical name of the plugs are National Pipe Thread (NPT) known as pipe plugs and the sizes start at 1/8 inch pipe for the oil pressure and generally 3/8 pipe for the water temp. but I have to say I don't think the radiator is a good place for the water temp to be connected to, usually located before the thermostat, this means it is usually threaded into the intake manifold near the thermostat housing. For normal automotive use the plugs range in size from 1/8 inch, 1/4 inch, 3/8 inch and usually 1/2 inch is the largest you will find in a standard car engine. They are available in hex head or allen wrench configurations what ever fits your needs. Generally the tube for the water temp is filled with gas that expands with heat thus moving the gauge,( not liquid). good luck , jeffco