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Rich;
Bad news...I think that may be your problem. Those pistons are the L78 style replacement pistons. While they are listed as "closed chamber" pistons, note that the chambers are not the same between rectangular port and oval port heads. The L78 pistons are known to just slightly hit the corners of the chambers on an unmodified oval port head. I've ran into this issue myself in the past, and another member on here ran into it a few years ago as well.
Read
this thread over on the Chevelle site.
Eric
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Boy, thats great news.. I hope this is not the issue.. Wouldn't there be more problems, or the motor would not run right if the piston was hitting???
Rich