METRIC STANDARD PARTS USE OF METRIC AND CUSTOMARY NUTS BOLTS AND SCREWS Some nrodel 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 car 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 lf they are not re usable care should be taken to select a replacement that matches the original General Motors Engineering Standards have adopted a portion of the standard metric fastener sizes defined by Sl 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 l 4 2O and 1 4 28 screws are replaced by the metric M6 3 x 1 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 CUSTOMARY METRIC 1 4 M6 3 Thread Major Thread Major Diameter Diameter in Inches in Millimetres 20 l Number of Distance Threads Between Threads per Inch in Millimetres Care should be taken when servicing the car to guard against cross threading or improper retention due to interchanged 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 of threads WAREHDUSING AND DISTRIBUTION DIVISION DF GENERAL MOTORS CORPORATION 89 337