Machine screws are sized based on several criteria. The diameter of the threaded shaft, the frequency of the threads on that shaft and the length of the screw are the most common methods for measuring screw size. The diameter and thread count of machine screws are measured using the same criteria for all types of machine screw. However the criteria for machine screw length depends on the configuration of the screw's head.
Machine Screw Length
Machine screw length is measured from the point at which the head of the screw is level with the surface it is buried into. The length of flat- and oval-head machine screws are measured as the overall length of the screw. However, pan- and round-head machine screws are measured from the bottom of the head to the threaded end of the screw. All four types are typically measured in inches.
Machine Screw Diameter
The diameter of machine screws is determined by the size of the screw shank. The shank is unthreaded portion of the screw below the head. For screws that are fully threaded the diameter is equal to the diameter of the threaded portion of the screw. Machine screws larger than 1/4 inch in diameter are measured by in fractions of an inch. Screws smaller than 1/4 inch use a numbered system that ranges from 4 to 14.
Thread count measures the number of threads the screw contains per inch. Smaller machine screws have a higher thread count-TPI-than larger screws. Machine screw thread count is commonly measured on a scale of 16 to 40. Sixteen is considered a very coarse thread and is found on large machine screws. Conversely, 40 is a very fine thread that is found on smaller machine screws.