Loud metal clash while driving, no longer starts

[digger]

my guess is its broken at the position that is just below the end of the guide. a non concentric (worn seat) causes additional eccentric loads that bend the shaft whenever the valve closes. after alot of cycles it fatigues, 300k is alot of cycles if they are original valves

[Chilezen]

my guess is its broken at the position that is just below the end of the guide.

You're a damn good guesser.



All the reused valves were ground. All 12 valve seats were cut by a machine shop.

I've heard the term "lapping the seats" but did not perform this. Another regret.

[digger]

could it also be that there was piston to valve contact and thats why it broke?

[Chilezen]

The piston is indeed cracked, but I haven't had time to take it out yet. With the timing belt fully intact (a Gates racing belt by the way), and the teeth lined up, I don't imagine timing was an issue. I don't know what else could cause contact, but I'd really like to know.