The steel studs pull out of the case, which is made of a metal less strong than steel. They are screwed into the case, and they strip the female threads tapped into the case. It appears this did not start happening until engines grew to 2.7 liters with the magnesium case.
Differential expansion is what does this - as the engine gets hot the studs and the cylinders expend. With early cylinders, which were either steel or aluminum clad steel, and thus expanded at the same rate as the steel studs, there was no problem. The problem arose after the switch to aluminum cylinders, which expanded more than the steel studs. In addition, the engines ran hotter because they were putting out more power, or because emissions stuff in the exhaust side heated up the cylinders more. These expansion forces eventually overstressed and fatigued the threads in the case.
The 3.0 engines and successors have an aluminum case, which is a stronger metal and thus more resistant to pulling the threads loose. In addition, with the 3.0s the factory began using studs made from Dilavar, an alloy whose expansion characteristics were more similar to those of the aluminum from which the cylinders were now being made. Plus the factory was able to use other methods than "thermoreactors" to meet emissions, and hence could keep engine heat down some. According to the authorities on such matters, all these "fixes" meant studs quit pulling with the advent of the SC s.
I have wondered if Dilavar studs are even needed with the aluminum case, though I haven’t been willing to use any of my engines as a test subject. Are the strong but expensive Racewear studs an alloy with matched expansion characteristics, or just strong?
In any event one doesn't hear much about head studs pulling in SC engines, so you can mark that off your list of things to worry about, I think. It is those of us who bought a 2.7 car, and then at our first meeting than we need to).
Walt Fricke
FrickeW@ci.boulder.co.us
