I am in the process of prepping my 1991 heads with 240000 miles for a valve guide and grind job, but before I dropped them off did I disassembled them to inspect. I found no visable cracks between ant of the valves but what I did find were cracks in 5 out of 8 precups at the corners of the hole leading into the cylinder. Are they ok to reuse, is this normal or should I not risk the reuse of these parts?