A wee bit more work over the last few months... stripped the engine down (again) and had a very close look at everything. As part of that I found a head nut was loose. That particular stud anchor had failed earlier, I'd helicoiled a repair, and it had seemed fine on that earlier reassembly. It wasn't any more... The thread supporting the helicoil in the casting had completely torn out.
I'd drilled and tapped by hand. The stud hadn't been perfectly aligned. The under-tension part of the helicoil was only as long as the stud's threads, into soft casting alloy. It's possible that the helicoil had an issue with neighboring threads twisting relative to each other, since it's essentially a rolled-up spring. The thread profiles aren't particularly well defined.
I turned up a solid bung, internally and externally threaded, with a much longer engagement length than the helicoil. It's nothing fancy in terms of material, just some mild steel. That's deliberate. If anything in the chain fails, I want it to be this bung rather than the engine casing again.
The other part to the job was getting the relevant hole drilled on center and straight. I made a special tool for this, to guide both the drill and the tap. The idea is to transfer accuracy from the machine tool to the engine in its stand, then work from there.
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