Normally, there is a small step in the combustion chamber that slightly shrouds the intake valve. When the head is milled, unless the valve is recessed the same distance, the step goes away and exposes the edge of the valve to the flame front as the fuel burns. Obviously, the alleged "machinist" didn't know to move the valves back after the milling was done. That's another reason to find a different metal butcher!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!