Your frustration is understandable, justified and needs to be vented out. It's the sheer "unnecessariness" of it and how EASILY It could have been avoided. At such times it feels almost as if the other guy wanted to do it deliberately. This s NOT the sort of "patina" you want on the car.
You're very right about these old GM engines but my share of holed pistons came about on the XJ's original engine when it dropped a valve during just idling. Am sure THAT was bec the head had been skimmed just once too often. I always found these engines a bit rough. The 2.8 version was even worse but the 3.4 revved a lot better, felt smoother.
The Buick is sort of mothballed at the moment. It's an all original 1974 Electra Limited with the 455cu:in ( 7.4litre) very torquey but still rather all f#^t and no s%^t engine. Plz excuse the French but a better way of describing this isn't coming to mind just yet! It is one of the 4 ( I believe) that were specially airlifted along with 6 LeSabres on an emergency basis for the Islamic summit conference in 1974 bec, as the story goes, Mercedes were not committing to supply the stretched 115s.
I managed to get hold of it in 1993 when it was languishing out in the open in a workshop here in Lahore. At the time it belonged to Peshawar-based film producer and has apparently appeared in a few Pushto movies!
Am sending a picture of it taken a few months ago. It's currently still jacked up and I intend to get it back on the road with some gentle refurbishing during the next summer, much on the same lines as what you have done on the jaguar. The OTHER picture is the main reason for putting it up on bricks as I can't possibly manage both at the same time !!!!

