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While I can't state for certain that it is "by design" it certainly makes sense from a consumer's point of view; bottom-line replacement cost in the event of a failure.
All materials yield, even Grade 8 steel is only 150 ksi capable. I guess ARP may even harden beyond Grade 8 requirements but steam pressures dwarf ordinary diesel compression. Steam drives locomotives with pressures in the range of 350 to 1,500 psi. The 6.0L has an 18:1 compression ratio. That equates to a minimum of an additional 6,300 psi on the head at the piston (95mm diameter, about 11 square inches) resulting in - get ready for it - an extra SIXTY NINE THOUSAND THREE HUNDRED pounds at each piston of which there are eight with ten bolts securing the head for an additional whopping 55,440 pounds on each bolt/stud. Grade 5 bolts are capable to 120ksi in tension while Grade 8 are capable to 150 ksi in tension. Keep in mind the steam is an additional load on top of the load created by compression ignition of diesel fuel.
The additional load from the upper limit I used in the example produces 237,600 pounds on each bolt/stud. It is very easy to see how bolts and even studs fail from STEAM introduction to the combustion chamber.
Even a safety factor of 2 using Grade 5 bolts (and the torque to yield bolts might only be Grade 2) would ordinarily ensure bolt failure before head deformation providing a salvageable engine with usable heads ($1,500 each I believe).
Makes sense from an engineering design point of view too, mathematically speaking.
Jonathan D. Howell
Lieutenant Colonel, U.S. Army
2005 EarthRoamer XV-LT (Ford F550)
"Americans Travelling America"