1997 GMC C7500 Topkick Engine: 6.0L V8; Standard Rotation; Severe Duty Thermal Fan Clutch; w/ 6 Bolt Mounting Holes; Marked NG or NF;. 1997 GMC C7500 Topkick Engine: 7.0L V8; Standard Rotation; Severe Duty Thermal Fan Clutch; w/ 6 Bolt Mounting Holes; Marked NG or NF;.