99 Northstar. 108K, drove it in the garage with suspected blown head gasket (1 & 2 were blown); however Cylinder 1 has vertical grooves and little dimples in the lower part of the piston and head, as if something sharp was tumbling inside for a while (a second). No valve damage. Grooves may be 10 thousands deep. Was this a piston ring fragment? Pull #1 piston? Is this block worth salvaging? If so, what steps should I take?

 

With a feeler gauge I can at least tell the top piston ring is in tact. Unable to feel any gaps. Actually the grooves in the cylinder wall are nowhere near 10 thousandths, but easily detectable with a fingernail.

 

With a feeler gauge I can at least tell the top piston ring is in tact. Unable to feel any gaps.