I would pursue the fact that it is always #4 or #1, myself, but that's just me - I am sure you know what 4 and 1 have in common.
I don't think I would chase the stud avenue.

 

Can you please elaborate on this?

 

Was there any resolution this issue? I am trying to research head gaskets, and would be very interested in what was the cause of these failures.

One thing that came to my mind is that perhaps the head has overheated badly and therefore lost the heat treatment and the hardness that comes with it. Another is that the block is somehow cracked in a way that allows / causes the cylinder towers to sink when warm. But take these as just uneducated guesses -- I am seeking advice and incompetent to give it.

 

It wasn't a Porsche engine, but I had 4 cyl. I/O boat engine that leaked water into one of the cylinders. Thought it was the head gasket - it was fine, had the head magna-fluxed for good measure to check for cracks - fine, even put the offending cyl at BDC and filled cyl with 150 psi of air and left overnight - still 150 psi in the morning. Finally found a small hairline crack in the cylinder wall that only leaked water into the cylinder once it started to warm up. Ran fine for the first 5-10 min. then started to run like crap. Ended up getting a new short block. Hope this is not your problem, but food for thought.

 

my will warm up to barely above the half mark when sitting at a stoplight on a hot day. If you rev it a little the temp will go back down. It is common for these car to warm up when sitting. They do fine when moving though