Hi there,
The only other thing that comes to mind is valve bounce, This can happen with over revving, or a weak or broken valve spring. This is a localised problem, one valve striking the piston. So something has caused the valve to stay open longer, Check the valve lift with a dial indicator, compare with another exhaust valve, check spring pressures, there will be something amiss there.
Mark (mhpautos)
Saturday, January 12th, 2008 AT 10:20 PM