Thread Inserts are essentially 'adapters' to go from one thread size to another. They are made of steel for strength. Thread Inserts part #'s HD-T1250S, T1375S and T1500S have internal and external threads through their entire length. Thread Inserts part #'s HD-T1625S, T1750S, T1875S and T1100S have full external threads but the internal thread only runs part way through with a hex socket for hex or 'allen' key tools to drive.
All Thread Inserts have 10-24 Internal Threads and 5/16-18 External Threads.