Big D,

I have enclosed a link to a picture of the thrust plate on the cam. It's hard to describe how it works...the thrust plate is screwed to the block and it holds the cam in place...the thing is, the thrust plate is captured on the cam between the cam gear and a shoulder. If the cam gear isn't up almost tight where it belongs, the thrust plate isn't captured.

Trying to pull the cam gear on with the two screws that hold the thrust plate would merely push the cam out the back of the block. Look at the picture and it will make sense..I hope.

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0631.htm

Stuart