GM54 USE OF METRIC AND CUSTOMARY NUTS BOLTS AND SCREWS Some model cars present special service requirements to the technician due to the use otlrotlt metric and cttsttnnary inclit type nuts lvolts and screws on the same car Many are metric and some are very close in dimcnsttnt to tsnstoinary nuts bolts and screws in the inch system l ltsmatched or incorrect nuts bolts and screws can result in damage tnaltunctittn or possible personal injury Nuts bolts and screws removed tronr the car should be saved lor re use whenever possible lt they are rot re usable care should he taken to select a rcplnccinent that matches tlte original General Motors Engineering Standards have adopted a portion of the standard metric fastener sizes defined by Sl t Systeme lnternational This was done to reduce the number oi sizes used and yet retain the best strength characteristics in each thread size For example the customary H4 20 and l 4 2B screws arc replaced by the metric M6 x l screw which has nearly the same diameter and 25 4 threads per inch The thread pitch is in between the customary coarse and ine thread pitches Metric and customary thread notation differ slightly The difference is illustrated below CUSTOMARY METRIC tt 4 M6 Thread Major Thread Major Diameter Diameter in Inches in Millimetres Z0 l Number of Distance Threads Between Threads per lnch in Mtllimetres Cate should be ta ken when servicing the car to guard against cross threading or improper retention due to intercliangcd metric and inch nuts and bolts When obtaining metric or customary nuts bolts and screws locally for servicing the car care must be exercised in selecting parts that are equivalent to the original parts in dimensions strength and pitch ot threads WAREHOU5tNG AND DtSTRlBUTlON DIVtStON OF GENERAL MOTORS CORPORATlON CHEVROLET tl REV 5 B1 M 1