USB OF METRIC AND CUSTOMARY NUTS BOLTS AND SCREWS Some model cars present special service requirements to the technician due to the use of both metric and customary inch type nuts bolts and screws on the same ear Many are metric and some are very close in dimension to customary nuts bolts and screws in the inch system Mismatched or incorrect nuts bolts and screws can result in damage malfunction or possible personal injury Nuts bolts and screws removed from the car should be saved for re use whenever possible If they are not re usable care should be taken to select a replacement that matches the original j General Motors Engineering Standards have adopted a portion of the standard metric fastener sizes defined by SI Systeme International This was done to reduce the number of sizes used and yet retain the best strength characteristics in each thread size For example the customary 1 4 20 and 1 4 28 screws are replaced by the metric M6 3 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 fine thread pitches Metric and customary thread notation differ slightly The difference is illustrated below V CUSTOMARY METRIC 1 4 Me 3 Threacl Major Thread Major Diameter Diameter l in Inches in Millimetres 20 1 1 Number of Distance Threads Between Threads l per Inch in Millimetres Care should be taken when servicing the car to guard against cross threading or improper retention due to j interchanged metric and inch nuts and bolts When obtaining metric or customary nuts bolts and screws locally for servicing the car care must he exercised in selecting parts that are equivalent to the original parts in dimensions strength and pitch of threads GM PARTS D vts ON GENERAL MOTORS CORPORATION cHEvROLET 14 REV 3 79 M 1