Starrett 6 Screw Pitch Gage With Locking Device, 4-42 Threads Per Inch, 30 Le...Narrow design blades permit use for internal and external screw threads. Features a locking device at each end to secure all blades for storage or position an individual blade for use. Correct pitch is determined by matching blade teeth with threads. Each blade is clearly marked with proper thread size. Blades are protected in sturdy steel case.