Dave,
0.647 isn't in my 1951 book or in anything newer than 1956 (I have a gap from 51-55). Seems all of the slingers shown in 0.648 are for V8's as mentioned above.
Maybe someone deleted 0.758 and got there wrist slapped because Chevrolet still had inventory and they reinstalled it as a new group (grabbing at straws here...lol).
So, is there another group that we haven't discovered...why do these 6-cylinder engines have slingers but they don't show up in the parts books???
Mike B
