[Flieger]

And a mean tensile stress reduces the fatigue endurance limit. So I would want to have the minimum preload (at any temperature) to be just enough to overcome the cylinder pressures and keep everything from fretting.

Or were you referring to the Aluminum fatigue? Well, Aluminums do not exhibit a fatigue endurance limit and even if they are in compression the polycrystalline nature will mean that there will always be some grains oriented in a bad direction that will be loaded in shear or tension which can start a crack even with cyclical compressive loads.