There are several tap drill sizes used today. The BSP (British standard pipe) consists of thread standards that are used on pipes with female and male ends. Two types are available, tapered and parallel.
These threads have similar sizes through the course of the length thread. In contrast, the diameter of a tapered thread varies from the bottom to top. The G set are the parallel standards. The R set is for the tapered variant.
Tapered threads have an “Rc Size ISO” mark. The parallel types have a “G Size ISO” mark. Standards don’t exist for Rc tapered tap drills for the 1 3/4-inch, 7/8-inch and 5/8-inch thread.
The BSPP (BSP parallel standards) are 13. They are 57 mm for 2-inch thread, 51 mm for 1 3/4-inch thread, 45.5 mm for 1 1/2-inch thread, 39.5 mm for 1 1/4-inch thread and 30.75 mm for 1-inch thread.
Other standard tap drill dimensions are 28.25 mm for 7/8-inch thread, 24.5 mm tap drill for 3/4-inch thread, 21 mm for 5/8-inch thread, 19 mm for 1/2-inch thread and 15.25 mm for 3/8-inch thread. Also used are 11.8 mm for 1/4-inch thread, 8.8 mm for 1/8-inch thread and 6.8 mm for 1/16-inch thread.
Tapered Tap Drill Sizes
Ten standards are used here. They are 56.75 mm for 2-inch thread, 44.5 mm for 1 1/2-inch thread, 39 mm for 1 1/4-inch thread, 30.4 mm for 1-inch thread and 24.25 mm for 3/4-inch thread. Other standards are 18.75 mm for 1/2-inch thread, 15 mm for 3/8-inch thread, 11.5 mm for 1/4-inch thread, 8.6 mm for 1/8-inch thread and 6.6 mm for 1/16-inch thread.
Factors that Determine Size
One of them is the bolt or screw diameter. Other factors that come into play are the tap percent and the threads per inch. The bolt / screw diameter is the material’s diameter. The pilot hole dimensions have to be smaller compared to the drill. This is necessary for the treads to engage.
Threads per inch will range from extra fine to coarse. The finer threads are used for applications that demand precision. Fine threads have plenty more threads per inch compared to a coarse one. Coarse types are utilized in general all purpose tasks. Taps use 75% thread or 50%, depending on the application.
As these facts show, tap drill sizes have to be evaluated carefully before being used. A thorough study of the application is needed first.