v8volvo: Thanks for the kind words. At times this seems an odd windmill to be tilting at.
I have been accumulating the proper tools and information, unfortunately in this case I left the most important one (the metric dial indicator) at home nearly 700 miles away. This is a Lemons car, so tools don't count toward rules budget (just actual budget).
I started with an unknown engine, and then we ran it in an unknown state for some time (3 races?) while it was "fine."
At the race before the pistons and rings (Buttonwillow 2018), the blowby was causing a visible fog above the idling engine (open breather, no recirculating into the intake). It also liked to put all the oil onto the outside of the engine in quite spectacular fashion. Hence the decision for a light refresh. Pistons and rod bearings just because I was in there anyway and it seemed prudent to replace them given the amount of work to do rings.
I probably should have sent the engine to the machine shop, but ran out of time. One of the bores had suffered some significant trauma in the past and should have led to an overbore.
It's encouraging to hear that oil can linger in the coolant passages. We'll work on that this weekend. I was thinking dish soap and running it, glad to hear I'm not alone.
Another source of unreliability is the human factor. It's not just me driving the car, and I fund some of this by renting out seat time (generally referred to as Arrive and Drive in the niche) to decidedly non-professional drivers. So instructions are sometimes not followed, and sometimes there are racing incidents (unrelated to the engine) and sometimes there are car problems unrelated to the engine.
For example, we were initially sidelined for hours at Buttonwillow 2019 to replace vent hoses on the fuel tank. The ones from 1984 had turned to some sort of crumbly hard substance that wasn't at all fluid proof anymore. The fuel tank on a 924 sits above the transaxle (because, as near as I can tell, they were designed to be disposable and assembled upside-down), necessitating some significant disassembly.
Then we spent much of Saturday at Houston 2019 (a month and a half later) chasing fuel leaks that were related to the work done to fix the fuel tank and head. Someone had wrestled the filler neck with pliers and damaged the cap sealing surface, injector leak-off lines had come asunder and were spraying fuel everywhere, etc. Then the wheel bearings failed and undid the spindle nut on the left rear and the wheel (and drum) fell off.
It's just racing. Stuff happens and you fix it, hopefully quick enough to get back out on the track. The extra head gaskets are talismans. One never needs the parts one has on hand.
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