Form of thread defined. Tables: basic sizes, limits, tolerances for 'class' bolts, nuts; BS Whitworth range, 1/8 in to 6 in; BS Fine range, ¼ in to 4¼ in (limits and tolerances up to 3 in only). Also a Selected Thread series more suitable than BS Pipe parallel series for extra fine pitch applications. Appendices: limits, tolerances for B.S.W. and B.S.F. truncated bolts and nuts; bases of tolerances and allowances, formulae; effects of pitch and angle errors, acceptability of error combination; form of Whitworth crests; limits and tolerances for (non-preferred) 'close class' nuts. Cross References:BS 21BS 919BS 1157BS 1580BS 1916:Part 2BS 2517BS 2779BS 3643:Part 1 BS 3643:Part 2Handbook No. 18Incorporates the following:PD 5902 published 11 August 1966