Parallel pipe threads are of equal size throughout the length of the thread. Tapered pipe threads change diameter from top to bottom. BSP standards for parallel threads are the "G" set of standards. BSP standards for tapered threads are the "R" set of standards. Parallel threaded taps are marked, "G Size ISO." Taper-threaded taps are marked "Rc Size ISO." There are no BSP standards for Rc tapered tap drills for 5/8-inch, 7/8-inch or 1 3/4-inch thread.
There are 13 BSP parallel (BSPP) standards: 6.8 mm for 1/16-inch thread; 8.8 mm for 1/8-inch thread; 11.8 mm for 1/4-inch thread; 15.25 mm for 3/8-inch thread; 19 mm for 1/2-inch thread; 21 mm for 5/8-inch thread; 24.5 mm tap drill for 3/4-inch thread; 28.25 mm for 7/8-inch thread; 30.75 mm for 1-inch thread; 39.5 mm for 1 1/4-inch thread; 45.5 mm for 1 1/2-inch thread; 51 mm for 1 3/4-inch thread; 57 mm for 2-inch thread.
There are 10 BSP tapered (BSPT) standards: 6.6 mm for 1/16-inch thread; 8.6 mm for 1/8-inch thread, 11.5 mm for 1/4-inch thread; 15 mm for 3/8-inch thread; 18.75 mm for 1/2-inch thread; 24.25 mm for 3/4-inch thread; 30.4 mm for 1-inch thread; 39 mm for 1 1/4-inch thread; 44.5 mm for 1 1/2-inch thread; 56.75 mm for 2-inch thread.