There were at least 5 different factory design cast impellers used on late '63-81 Pontiac waterpumps.
-abbreviated style impeller used on '64 & 65 8 bolt pumps.
-longer vane impellers used on '66-68 8 bolt pumps
-early shallow recess impellers used on the first 11 bolt pumps
-late deeper recess impellers on the last casting of 11 bolt pumps
-abbreviated vane used on 301 & 265 pumps (utilized as the hard block 400-455 impeller)
-John V has mentioned an even less drag (shorter height vane) cast impeller used in a '63 Tempest 4cyl pump. I have not ran across one of these & have removed numerous original casting '63 8 bolt pumps & compared them to other early pumps with their original abbreviated cast impellers.
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