Drilling and Tapping threaded holes.

Because the thread angle is 60degrees... and the COS of 60 is 0.5, each side of a bolt's thread is 1/2 of the pitch distance. It means you just subtract the thread pitch from the bolt diameter to get the pilot size. It means that a M5 x 0.8 bolt has a pilot hole size of 5 - 0.8 or 4.2.

Thanks to 60 degree threads, you take the major diameter and subtract the threads per mm or threads per inch to get the pilot hole diameter.

Metric is easy... M6 x 1.0 is 6 minus 1 = 5mm pilot hole.

SAE... 1/4-20 thread. Convert 1/4 to decimal. Convert 1/20th to decimal. Subtract 0.25 - 0.05 = 0.20 . Go to drill index and find there is no 0.20 drill but a Number 7 drill is 0.201. Easy! right?????