1975 GMC G15 Engine: 4.1L L6; Standard Rotation|Optional Non-Thermal Fan Clutch|w/ AC;. 1975 GMC G15 Engine: 5.7L V8; Standard Rotation|Optional Non-Thermal Fan Clutch|w/o AC;. 1976 GMC G15 Engine: 4.1L L6; Standard Rotation|Optional Non-Thermal Fan Clutch|w/ AC;.