've had this problem before, The exhaust timing gear may have spun causing the pistons to crash into the exhaust valves. The problem was preceded by what sounded like valve clatter. all 8 valves ended up bent enough to not let them close. The timing chain will still look like its on the marks. The gear may even fall back into place. If you remove the gear you can see a groove where it spun. I noticed this after taking the valve cover off and seeing that none of the buckets over the springs were rising all the way up in comparison to the intake buckets,