Yes that is shocking Steve, was it on all cylinders or just one?
It only occurs normally on the back (no 6) cylinder.
This is the 3d or 4th head I have seen with the same symptoms.
These long L6 heads do not cool so well towards the rear so excessive temps can be seen. This adds to the risk of "det".
The waterways get clogged up over time with corrosion sludge and sometimes old casting sand coming loose, thus reducing flow in the head particularly at the rear.
It is one of the reasons why Skiddell, Z bloke and myself go to the trouble and expense of having blocks and heads hot Caustic dipped or acid dippped before building a quality engine to ensure we maximise water flow/cooling.
We were fortunate at one stage in being part of an experiment involving multiple sectioning of a head to try and understand the "flow problem"
We have highligted this issue as good unmolested undamaged heads are becoming harder to source.
The head that Steve pictured is beyond economic repair at current values.
Hoever the desirable N42 we have with a similar but less severe issue has been put to one side till I can get our machinist to repair it
What sort of mileage had that engine got on it Steve?
I only acquired the head in a job lot purchase there was no block/bottom end.
Why not ... well given the severity of head damage one could assume with some confidence that the det destroyed the piston crown with catastophic results for all the rest of it. Speculation of course but very possible.
So if you are having uncontrolled explosions inside your engine its not good and needs fixing while its still fixable