Thermal expansion coefficients are higher with aluminum and its alloys than with cast iron, but that does not absolve the designers from compensating for the coefficient differences in the design. Including the gaskets. and including thermally induced cold flow of the gasket material. Retorqueing headbolts after purchase is not required of any other engine and should not be required of Subies either. Modern computer programs have built into them the capability to do all of the thermal mapping, finite element analysis, and everything else necessary for engineers to have designed this problem into history… retorqueing the headbolts is just a bandaid, not a fix.
The problems resulting from going from eight headbolts to six headbolts should not have happened. Not with the design programs that have been in existence for the last 30 years. There’s no acceptable excuse.
I’m fully aware that it happens with all materials, and if I still had the ASME standards lined up next to me in a technical library I could tell you exactly how much for each material. If you think problems from it are impossible to avoid in an automobile engine, you should have been part of our team that designed active sensors that mounted on the afterburner shrouds of the engines that went in the F-15s and F-16s. We were dealing with extreme temperatures combined with extreme vibration. And the components included not only steel and aluminum, but polyimide-amides, silicone silastics (yes, combined with glass spheres they can be made to withstand high temperatures), a high temperature plastic called Torlon, and even a hermetically sealed tube with a fused quartz bulb sensing through a fused silica lens.
I lay the fault with the engineers. No place else. They were not “pushing the envelope”, they were designing a normal engine, albeit with a different cylinder layout. That’s not to be interpreted as meaning that I dislike Subies, because I don’t. It only means that I consider the headgasket problems to have been solely on the shoulders of the engineers.
By the way, thanks for bringing back some great old memories. That was a truly great team we had. The best bunch of guys I’ve ever worked with. Some of us received formal awards from Pratt & Whitney’s design group out of Florida for our parts in the design, but the truly great thing was the guys. A more eclectic group I’ve never seen.