GMC "Why Preventive Maintenance" reproduced, booklet prepared by... GMC Truck and Coach. Book explains recommended preventive service.   Maintenance for Light Duty A, Heavy Duty A, B, C, D, E service worksheets shown. Read explanation for "WHY" lubricate a chassis, tune an engine, align front wheels, inspect a generator, clean the crankcase ventilator, remove wheels, check the drive-line, and much more. Contains illustrations.