Ford days it's a spark plug problem, not a head problem.
Well I believe that when my poo turns purple and smells like rainbow sherbet.
From a purely engineering standpoint, my understanding of how this was developed, and why it failed is as follows:
- Ford previously had full-thread holes in the heads. This was a PITA to get the plugs to centre when being installed on the assembly line so it was looked at how they could improve the self-seating ability of the plugs in the holes for fast turn-down on the line.
- Research was performed and the results indicated that sufficient engagement was provided by ~3 threads, this meant that the remainder of the hole could be bored smooth so that the plug could be dropped down the hole and tightened down extremely quickly, since the smooth section set the plug straight.
- Research was NOT done to explore the behaviour of this setup once damage to the threads had taken place due to removal/installation; there was no testing as to how this would fare with in-the-field service.
- So from a design standpoint, the 3-threads were technically "sufficient", however, as we all know, the risk of damage to those threads on a subsequent long-interval removal and install is extremely high. It doesn't take much galling to compromise the integrity of this setup and result in a loose plug, which may in turn become a blow-out.