i've been breaking in my 283, got about 400 miles on it and at 45 miles an hour clack, clack, clack. i cut it off quick and rolled to the side of the road. WTF? i gave it another try to figure out where the sound was coming from, sure enough, pulled the valve cover off and the valve was sheared in half up by the keeper which i found both halves. the valve didn't have enough room to hit the cylinder walls but hacked up the piston pretty good. i pulled the head this morning. should be warrantied by the engine builder, but what would cause this? i'll drop the oil pan to make sure there isn't any bits of stuff down there and make sure the oil passages are clear, i guess i'll bring the motor guy a head and a piston to be fixed. i don't want to send the whole motor it took him a year and a half to build it the first time.