A tap is a device for adding female threads onto a bolt. Tap sizes range from 0 to 14 with 0 being the smallest tap in exterior diameter. Each tap must meet limits and tolerances for the specific job it must complete. These limits and tolerances can very, creating subclasses within sizes of taps. For example, Size 2 taps may be either 2 or 2B based on the degree of tolerance required. There are many differences between a 2B tap and a 3B tap.
The sizes of a 2B and a 3B tap are different. The 2B tap is smaller in size. It has an outside diameter of 0860 inches. The 3B tap is larger by .0130 inches, making its outside diameter .0990 inches. As the sizes progress up the tap scale, it increases in size by .0130 inches in outside diameter. Other types of taps, such as hand or fractional taps, increase in increments of 1/4 of an inch and referenced not as a number, but the size in inches.
Screws and Nuts
The screws and nuts to be used in association with a tap must also match the size of the tap. For example, a 2B tap must be associated with a 2A screw and a 2B nut. The screw, which has the male threads, receives the A designation and also references tighter tolerances than a Class 2 screw. The 3B tap must be paired with a 3A screw and a 3B nut. Mismatching these will result in shallow grips on the screw which can be easily striped.
Each size of tap has certain purposes or uses. Other taps are not utilized very often. For example the 1 tap is not used in modern metal working. The 2B tap has many different uses. The 2B tap is used in plating, finishes and coating according to Newman Tools website. The 3B tap is not used as commonly and is utilized in cases where the tolerance levels are close.
GH levels refer to the pitch height of the thread cut by the tap. The higher the GH number, the higher the tolerance. GH levels can vary between taps. A 2B tap with a GH-5 has a higher tolerance than a 3B tap with a GH-1 tolerance. The matching screw must also have matching tolerances. On the screw this is referenced as the GL of the screw.