This 640 is for 1984-1999 Harley-Davidson big twin models with large displacement motors with compression ratios of 10.5 - 11.5:1 or smaller motors with 11:1 - 12:1 compression ratio, does not require as much compression as the 631. For hydraulic tappet, spring spacing is required. These camshafts are also for 1954-'84 big twins with either the Hydraulic Conversion kit or the Cylinder Head Replacement kit for panhead engines. They are also replacement parts for Flathead Power P-Series and SH-Series engines. Since these kits and engines include 1984-'99 style hydraulic lifters, the larger late style roller diameter requires that cams designed for 1984-'99 models must be used to achieve correct valve timing. View Cam Specs Cam Specifications for 1954-'84 Big Twin Engines Requiring 1984-'99 Style Cams CamApplication Generator Part # Alternator Part #Valve Timing1 Open/Close Valve Duration Valve Lift Lift @ TDC2 Spring Spacing RequiredTappet Type Required Intake Exhaust Intake Exhaust Intake Exhaust 51374"-84" Stock or low compression 106-6028 106-6027 18 degree/38 degree 40 degree/18 degree 236 degree 238 degree .472" .163" .187" No Hydraulic 58574"-80" engines 8.5:1 - 9.5:1 CR 84" - 93" up to 8:1 - 9:1 CR 33-5131 33-5109 20 degree/45 degree 60 degree/20 degree 245 degree 260 degree .540" .172" .166" Yes Hydraulic 60093"-103" engines 9:1 - 10:1 CR 33-5132 33-5058 20 degree/55 degree 60 degree/20 degree 255 degree 260 degree .554" .177" .170" Yes Hydraulic 631High RPM Cam for 93"-103" Engines 11:1 CR 33-5078 33-5080 34 degree/61 degree 66 degree/29 degree 275 degree 275 degree .581" .359" .204" Yes Solid or Hydraulic 64093"-Up Engines 10:1 or Greater Compression 106-4639 33-5108 25 degree/60 degree 65 degree/20 degree 265 degree 265 degree .590" .205" .192" Yes Hydraulic 1Timing designation is function of zero lash @ .053" off base circle. Lifts based on 1.5:1 Rocker Ratio. 2Lifts at TDC are measured at the valve and are for reference only. Overlap valve lifts must be checked on assembled motor. S&S Part #: 33-5108