Grade A (pitch-line) and Grade B (grouped) measuring cylinders for checking external screw threads of Unified and ISO inch, Whitworth, B.A. and ISO metric form by means of pitch diameter measuring machines. Bases for the diametral tolerances. Relative merits of grades A and B. Measuring ranges, material and finish, form and general dimensions, accuracy and identification; tables of nominal diameters and associated P values for Grade A cylinders; limits of size for Grade B cylinders.Cross References:BS 84BS 93BS 427:Part 1BS 1580 BS 3643Replaced by BS 5590:1978 but remains current.Incorporates the following:AMD 2656 published 31 July 1978