Take a look at my experience with this same leak. I have about 200 hrs on since doing Lyc SI1290 and It's still bone dry
I consulted with usual engine gurus here and called Divco.
https://vansairforce.net/community/showthread.php?t=191165
https://vansairforce.net/community/showthread.php?t=191860
The only thing I did different that the SI called for:
- instead of using a reamer to clean up the bore I used 1/2" bore brushes of progressive grit (200-800). It probably didn't need it but it was hard to tell how clean it was.
- I used a P2 rather than the P1 through bolt right way based on the extra .001 the bore brush took off
- I lightly coated the bore and associated part of the thru bolt with Loctite 518 before installation
- Since I used bore brushes that have a cutting medium, I made sure the bore was flushed several times and I pre-oiled the engine with about 6 quarts of new pressurized oil to flush any contaminates that made their way between the bearing and crank
I suspect the reason the oversized thru bolt tends to work because a normal thru bolt may not have much of an interference fit. The aluminum bore grows larger than the steel thru bolt and the interference is lost at normal operating temps. If there's any case fretting that provides a path for oil, from the main to the bore, it will get past the thru bolt also.
New cases are doweled and o-ringed so they don't have this issue. Dowels provide rigidity and o-rings seal off the oil path.
Hoping my fix gets me to next overhaul...so far, so good.