Tapered pipe threads are actually much larger than you’d expect given the nominal diameter. 1/4 inch tapered pipe threads have a maximum outer diameter of more

Tapered pipe threads are actually much larger than you’d expect given the nominal diameter. 1/4 inch tapered pipe threads have a maximum outer diameter of more than 1/2 inch and 18 threads per inch. They will sort of work with a 1/4 inch straight thread but not by fully screwing in.