Very sad I was not here when the thread was made. If the car is a factory CNG or LPG I would also suspect the head gasket.
And if so, it is very likely the block is warped too. I've heard horror stories of our S,S2,SG and SG2 engines. Especially CNG and LPG. Tomorrow after cleaning my head gasket material as well as carbon ring from the block I will test for warpage with a Straight Edge, flat 8mm thick with an accuracy of 12 micrometers.
I suspect it will be.
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