Hypothesis time. Possibly another piece of this puzzle is the plastic coolant jacket spacer. Toyota installs these to keep the bore temperature even, they help with emissions, mpg, and probably cylinder wear.
From the SAE Paper 4/2005
“For reduction of fuel consumption, a new device “Water Jacket Spacer” which improves temperature distribution of a cylinder block bore wall was developed. In the case of a conventional cylinder block, coolant flow concentrates at the bottom and middle region of the water jacket. While temperature of the upper bore wall is high (due to high-temperature combustion gas) the temperature of the lower bore wall is low, since its only function is to support the piston. When the developed spacer is inserted into a water jacket, the coolant flow concentrates at the upper part of the jacket. As a result, cooling ability to the upper bore wall was improved and temperature of lower bore wall was increased, thereby reducing fuel consumption.”
In many of Toyota’s open deck applications where these are utilized, the upper portion of the spacer is open equally around the entire perimeter of cylinders, with the odd piece to direct flow or prevent a cool spot.
Here is the 2GR FKS.
View attachment 3965539
Now with the 3UR (and all others in the UR Series) the jacket covers almost the entirety of the intake side of the sleeve. There is roughly .25” of space between this jacket and the head, thus keeping the intake side of the cylinder warmer. The exhaust side is open.
View attachment 3965542View attachment 3965544
This also blocks/slows entry of the coolant into tiny cross drilled coolant passages that run between cylinders. Where this passage meets to head gasket/head is where the UR blows the gasket. There are two things listed as causing an MLS gasket to have the rubber delaminate, inappropriate coolant type (perhaps acidic falls under this), and running hot/overheating. Perhaps we have a hot spot(s) here due to the jacket?
This possibility stems from researching Supercharged 3UR engine failures. When they blow the ring lands, it’s always on the intake side right where this jacket slows coolant flow and insulates the bore. They either detonate here, or the ring gap is too small (not enough heat removal and running out room to expand, they butt up and pop!). That’s a hypothesis, too.
Anyway, if we look at the coolant jacket, it’s molded in a way where it looks as though the extra blocking is added on to the intake side. The seam mirrors the opening on the exhaust side.
View attachment 3965555
The between cylinders cross drilled coolant entry points sit just above where these nubs are located on the jacket. Either holes drilled here, or removal of the material outlined by the seam, will provide a direct fresh coolant supply from valley distribution points. More durability, but also more emissions.
View attachment 3965556
Hopefully this leads to something, worst case is it goes to the scrap bin.
