I've taken the head off my mates 2wd saff to change the head gasket and when I lifted it off all four pistons have damage from were the valves have come in contact with them at some point in the past probally from a timing belt failure.
He has only just bought the car and the head gasket and timing belt look fairly new, but he wanted to upgrade to a group A head gasket. The valves themselves look fine. My question is it ok to run the engine with this clipping damage?