System 21 System 21 specifies for interchangeable assembly with reference to functional size only. System 21 specifies that functional size must be measured at the maximum material limit within the length of standard gaging elements. System 21 requires that the NO GO functional diameter must also be verified. This can be accomplished by using a fixed limit gage such as a GO/ NO GO thread plug gage or a variable gage with functional elements such as thread rolls System 21A Metric threads per ANSI B1.18M System 21A specifies for interchangeable assembly with functional size verified at the maximum material limit using standard length gaging elements. This can be performed by using fixed limit gaging such as GO/NOGO thread plug gages or with variable gaging through the use of functional elements. System 21A also requires that the minimum pitch diameter (minimum material limit) be verified by inspecting two thread flank location over the length of the thread. This can be performed with a variable gage or a thread roll snap gage with pitch diameter elements. System 22 System 22 specifies for interchangeable assembly with functional size verified at the maximum material limit using standard length gaging elements. This can be performed by utilizing fixed limit GO/NOGO gaging or variable gaging with functional diameter elements. In addition System 22 requires the verification of the minimum pitch diameter (minimum material limit) over the full length of the thread. This can be performed by using variable gaging such as pitch diameter elements. System 23 System 23 specifies for interchangeable assembly with functional size verified at the maximum material limit using standard length gaging elements. Minimum pitch diameter (minimum material limit)  must be verified over the full length of the thread. System 23 includes all the requirements of System 22 as well as the independent verification of lead, flank angle, taper and roundness. www.threadcheck.com https://www.threadcheck.com/thread-gages-gauges/ https://www.threadcheck.com/tri-roll-thread-comparator/ https://www.threadcheck.com/gagemaker-thread-gages/ [email protected] 631 231 1515 800 767 7633