Moriarity: Im not a race engine mechanic, nor do I profess to know lots about the big block drive trains, but after reading all the responses of knowledgeable and well-intended people, I would recommend that you don't get too exotic in your diagnosis, unless the build specs on your engine suggest it should be in a NASCAR, rather than your street cobra. Check for binding. Find out, if you can, exactly what springs you have; often people buy too stiff a spring. Examine the valve stem head to see if it's been mushroomed. Show the spring to someone with expertise in their failure to see if the fractures support the metalurgy inferiority theory. Put a bore scope into the spark plug hole to check out the bent value theory, and look for marks on the piston top. Examine the edge of the
oil pan where it bolts to the block for any bulges. Not likely, but a rod cap could have come loose. Drain the
oil, and look for any metal. Keep it if found, and show it to a experienced mechanic. The simple answer is that there is too aggressive a spring in your engine. This happens a lot. Just my thoughts. I wish you the best! Bob