In short the issue was a sort of mechanic's "perfect storm", a combination of several rather unlikely factors that in concert led to head #1's failure (this gets technical real quick, so skip to pics below if you don't want to spin your noodle):
- Height of intake valve projection at rest (cam lobe completely disengaged) of head #1 was on the far end of the acceptable spectrum (~0.78mm), vs. the new head #2 which sits at nearly half that (~0.40mm and lower). This is the distance that the intake valves project out from the plane of the cylinder head mating surface at fully seated position (in valve seat) and thereby would be the additional distance the int valve is projected when fully opened
- Valve relief pocket depth of PD150 pistons and stock AHU pistons differ by .06mm, PDs being shallower (0.95mm vs 1.01mm on AHU pistons)
- While the valve relief pockets in the crown of the PD150 pistons are the same width as the AHU (or ASV) pistons (37.32mm), the cross-sectional profile of the PD pistons is ever so slightly different, and most importantly the juncture of the two planes of this profile are at roughly a 45* angle on the PD pistons, whereas this juncture is basically a 90* angle on the AHU/ASV pistons.
Here are some more details and pics of our process:
Measuring intake valve projection of head #1.
Measuring intake valve projection of head #2.
Intake valve projection values of head #1 written in sharpie on the int valves (hard to see in this pic).
Misc notes from our measurements: valve relief pocket depth of pistons comparison at top, and valve relief pocket diameter at the bottom.
For reference from A3 Bentley manual: 3-notch gasket gives us approx. .20-.30mm extra wiggle room which is more than enough.
Cyl. head #2 being assembled; injectors installed, glow plugs installed, coolant neck installed, etc.
Exhaust manifold/turbo being installed.
Block surface cleaned/de-greased and new head gasket installed.
Head #2 installed on to head studs, and then torqued to spec.
Intake manifold installed and the rest of the connections being hooked up.
Filled with coolant and now it's back to its wonderous life!