Small block Chevy engines have used three different timing pointer locations. Pre-1969 engines have the TDC mark 2° to the left of the keyway centerline. 1969-85 dampers have the TDC mark 10° to the left of the keyway centerline. 1986-92 dampers have the TDC mark 40° to the left of the keyway centerline. Professional Products' small block Chevy dampers use the 10° TDC position. 6.75" models have two TDC marks: one that is 10° and another that is 40° to the left of the keyway. If you have a pre-1969 vehicle, you can use one of the aftermarket bolt-on timing pointers for correct timing mark alignment.