Two things jump to mind.
Jumped timing chain punching a valve through the piston top or a serious case of pinging.
How do the valves look and what octane did you normally run?
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.