Interesting as I thought the recess hole in the head was now too small for the dowel to sit in stopping the head compressing the gasket.
But that's not the case ?
Both the head and block faces are true ?
The height of the hole is not a lot, it's around 10.5mm for me. Dowel height is 11.8, gasket thickness 1.6. But as I see there is space to drive the dowel down. didn't tried it yet though. I don't know what's the recommended clearance, but that 0.2-0.3 doesn't sound too much, especially given the different materials.
From here, it would seem the simplest way forward would be to remake the dowels....
Scepticism would seem prudent - given the work involved to get the motor out, again.
It may not be the (only) issue after all...?
Dowels will be handled for sure. But I'm not yet convinced that this was the root cause. As you say, I want to rule out everything.
Despite that I can now remove the engine in one day, doing it repeatedly it's not so fun.
The other hypothesis is insufficient surface finish. Though it's not an MLS gasket, but of course the machine shop hasn't given a Ra number.
I don't have my notes here but as I remember 0.4mm was taken off because of the heavy corrosion.
It's not the dowels. They have space to move further down in the block, and they do on a slight tap, so the heads can push them down if needed.
Also measured the gaskets. On the original ones the fire ring thicknesses are 1.60 +- 0.3, on the new one it's not consistent, around 1.65 on one side and 1.75-1.85 on the other. Either they weren't crushed the same way, or they already differed from the get go.
So I cleaned everything, with extra care for the stud threads and nuts. Roughened up the side of the washers facing the heads to avoid the bearing effect. Oiled the nuts and threads and put back the head I've removed reusing the gasket. Torqued down in three steps, waiting one hour between the steps. Everything by the book.
Filled up with distilled water and pressurized the cylinders. Same symptoms, air bubbles coming up in the thermostat housing. Already around at 1.5 bars.
Got new head gaskets from a local manufacturer. I know other Ghibli owners here using this gasket so it's tested. Made extra care to perfectly align the fire rings, for this I had to adjust the cutout around the dowels. But even the aftermarket one I've received from Italy had alignment imperfections. Having the edge of the fire ring in the combustion chamber can cause detonation issues.
Same process again. Same results... it has to be something else than the gaskets.
Strangely on the first gasket the fire rings are not crushed identically. Although I don't know their original measures, but right now the middle one is around 1.8mm, the two sides 1.85mm on the intake side, 1.65mm on the intake side. On the original factory gaskets I removed it's consistently 1.65mm everywhere. This is all just the LH side, didn't touch the RH head yet.
I'll do the pressure test from the other side and try to peek in with a camera checking where the water comes in. For this I have to close up all the hoses somehow.
The head slid down the studs without feeling any interference, but what if indeed a sloppy resurfacing causes the studs hitting the side of the holes because of non orthogonal compared to the main plane. But if the head slid down, that final tiny movements should be allowed, shouldn't they?
Have you measured the head thickness at either end?
I’d have thought it would have to be a fair way out to lock up on the studs, was it binding on the studs when you pulled it off, you might be able to see witness marks in the holes. A race engine I used for many years had one head stud hole drilled off square. You’d have to screw it in and then bend it to get the head on and it never caused an issue.
Have you checked that the block and head are flat since youve taken it apart for the second time?