The valves would bend on impact so would not cause damage.
Normally if a belt/chain goes it would mark a few pistons not all of them
Have you had any head problems?
Had the head skimmed previously by too much & the valves have hit all pistons. ??
It did overheat in September, the effects of that didn't become apparent for a few months. I did quickly do the head gasket a few months ago but it made no difference so assumed it's cracked or warped. Those marks were there then too.
A different head is going on tomorrow.
The head was skimmed about 6 years ago, but only minimally to clean it. No issues at all until last September when the coolant pump failed and it overheated.
They weren't there 6 years agoSo the marks have been there since you split the engine originally.
They weren't there 6 years ago