Thread TermsMetric and American threads both conform to the same profile, a series of equilateral triangles with the crests chopped off and the roots rounded.

External Standard Thread Profile The depth of the threads is 54.127% of the distance between threads, and the radius of the rounded root is 14.434% of the distance between threads. Another way of looking at it would be to say that 1/8 of the height of each equilateral triangle is chopped off the top, and 1/4 of the height off the bottom, leaving only 5/8 of the height available. (The height of an equilateral triangle is equal to the width times half of the square root of three; 5/8 of this is 0.54127.) The root diameter of the thread is the nominal diameter minus 108.3% of the pitch of the thread. This means that fine threads have larger root diameters than coarse threads, and thus larger tap drill sizes. For threading using a tap or die, most threads are not cut to full depth, but to 75% or so. The resulting threads are not quite as strong, but full depth threading is very hard on the tap or die. Threading on a lathe presents no difficulty cutting to full depth. Thread SpecificationsThread specifications are written thus: 1/2-13UNC-2 which means: bolt diameter threads per inch thread type fit class There are four Fit Classes, ranging from falling-off-loose to scientific-instrument-tight. Class 1= Loose Class 2= Free Class 3= Medium Class 4= Close The class is followed by an A for external (screw) threads and a B for internal (nut) threads. Most are class 2. 3 is for precision assembly, and 4 is used for things like lathe lead screws and measuring instruments