1979 GMC C1500 Engine: 4.1L L6; Standard Rotation; Standard Duty Thermal Fan Clutch;. 1979 GMC C1500 Engine: 4.8L L6; Standard Rotation; Standard Duty Thermal Fan Clutch; w/o A/C;. 1980 GMC C1500 Engine: 4.1L L6; Standard Rotation; Standard Duty Thermal Fan Clutch;.