BS 57 BA Screws, Bolts and Nuts

BS 57 specifies British Association (BA) screws, bolts and nuts. The schedule below reproduces selected preferred sizes for cheese-head screws.

Schematic side and top views of a BS 57 BA cheese-head screw with head and thread dimensions labelled

Selected dimensions for BS 57 cheese-head screws

BA size	Thread major diameter D (mm)	D (in.)	Head diameter A max. (in.)	Head diameter A min. (in.)	Head depth B max. (in.)	Head depth B min. (in.)	Under-head radius R min. (in.)
2 BA	4.7	0.185	0.319	0.309	0.130	0.123	0.015
4 BA	3.6	0.142	0.252	0.242	0.101	0.095	0.010
6 BA	2.8	0.110	0.194	0.184	0.078	0.073	0.010

Note: Dimensions are selected values from the cheese-head screw schedule; all inch values are in inches. The standard includes other BA screw, bolt and nut forms and sizes.