The new 401 ran good and sounded great for a day or two and about 300 miles then it began to knock badly. I took it to the machine shop where the motor was done and he agreed to pull the pan and check things out. He found nothing until he started cancelling spark from each cylinder. He found that with no spark to the #7 the knock was gone. He said that it has a collapsed piston in #7. What is could be the cause for this and what should I look for when I pull it out to investigate the cause?