PD150 pistons and Rosten rods in AHU

Well, we're not quite to the bleating trumpets phase of this conversion like we thought we would be by today.  We are in the process of sorting out an issue that surfaced when we first started the engine on Fri.  Right off the bat it didn't sound right so we were carefully monitoring it at idle for a few minutes until shutting it down for further investigation.  And I'm glad we did that. 

For those who don't know this engine was a bit of an experimental engine build.  Up until this point we've always built AHU engines up with either stock AHU pistons (in our early days) or ASV pistons from later ALH engines (better oil cooling galleys in piston).  The engine for this conversion was a custom build using an AHU block and crank, PD150 pistons from a european TDI pump duse engine, and Rosten high performance rods, with a rebuilt AHU cylinder head fastened down with ARP head studs.  All bearing clearances were checked with plastigage during the build-up and were well within spec, and piston height projection was measured as well and the appropriate head gasket used (1-notch).  Everything looked good on paper, but it turns out we had two hydraulic lifters fail due to excessive pressure from below.  After pulling the cam and then the head off, we're certain that the pistons and rods are undamaged, but we're still a bit puzzled by what caused the valve "kissing".  A minor setback for sure, but we're busy at work to solve this issue and complete this conversion.  While we work out the details of this here are a few pics to chew on:

  Camshaft removed along with lifters.  #3 intake lifter (5th valve from L) is the one that had the top come off (not in pic).

View of pistons with head removed.  No evidence of impact damage, just the slightest imprint in the carbon of the intake valves at the periphpery of the valve relief chamber.

Head sealing surface also shows no signs of impact related damage.  Carbon residue here is normal and was lighly cleaned with brake cleaner for pic.