BS 7121-3:2017 specifies the essential dimensional features of keystone rings made of steel, types T,TB, TBA, TM, K, KB, KBA and KM, having diameters from 70 mm up to and including 160 mm, used inreciprocating internal combustion piston engines.Cross References:BS 7121-2-1:2012BS 7671:2001BS EN 13586:2004BS EN 14502-1:2010BS 7121-2-5:2012BS 7121-1:2016BS 7121-2-3:2012BS EN 14502-2:2005 ISO 4301-2:2009 Ed 2BS ISO 11660-2:2015BS EN 62305-1:2011ISO 4301-1:2016BS EN 13155:2003PD 5304:2014BS 8437:2005ISO 4301-5:1991 (R08)BS 6187:2011BS EN 13557:2003BS EN 13000:2010+A1:2014ISO 12480-1:1997