3/8" is the pipe ID size.
A pipe threads is a taper threads, it a interference fit. Like galvanized water pipes and gas black pipes that feed your home is this way. You should use Teflon tape on the threads to reduce the friction while tightening. All of the plugs and heater hose fittings on your engine is this way.
A bolt and nut has straight threads can be spun on the full length of the threads.
For more info:
https://www.engineersedge.com/hardwa...pe-threads.htm