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I'm glad I listened to a very experienced engine rebuilder
But actually I had no choice but to follow Neil's advice, since it's his shop
The big end bearing on cylinder 4 was indeed on its way out, meaning the spare long motor project is getting a full set of bearings.
It was obviously using oil, as witnessed by the pooling in the throttlebody & pipe.
And that was confirmed by Neil as he pointed out that the oil control rings are fully glued to the pistons, with varnish.
At this point he will tolerate whatever mods I want to add to the rebuild, but as Owen has pointed out repeatedly, the health of the rebuilt donk will almost entirely be down to having an excellent tune, to which we'll add supporting mods and correct break in procedure inc pre-priming the oil, a ceiling on boost and revs, but otherwise "hard" with cylinder pressure and vac to seat the rings properly.
My guess is the car the spare engine came in had a soft break in, used more oil than the standard service interval supported, overheated at some point (oil is an essential part of the cooling system after all) and the rest is history (head gasket issues).
I call this downward spiral the "service abuse effect" and my own car is an exemplar. It's been a major bugbear since we bought it, and although a great tune improved the reliability factor, the service abuse, combined with age, has lead to the inevitable conclusion of reco or replacement of its engine.
It's plain to see why a fair proportion of buyers of second hand Libs run into trouble, especially once they add mods and especially when done with either a crap tune, or no tune at all.