Your intake valve is really clean. Most times, there is a carbon build up on them. Same with your pistons. what you are looking for is small potholes in the top of the piston and cracks around the edge. When a engine has detonation bad enough. It will turn the tops of the pistons into silly putty and melt a hole through. Look at some of the pictures on google.
https://www.google.com/search?q=engi...ih=578#imgrc=_