When I was a bike mechanic in the seventies, I had a guaranteed method of getting mangled case screws undone. They were invariably dead tight and frequently undisturbed from the day they left the factory. Being reliable devices, they might be in place and temperature cycling every day for years.
Anyway, I would take a flat faced drift and place it on the face of the screw, striking it one hard blow with a heavy hammer. This reformed the mangled head of the cross headed screw and loosened the thread all in one go. I had a tool kit screwdriver onto which I'd brazed a tee bar, which I could tap into the screw , getting a good grip as it made its own shaped hole. They were always finger tight by this time although very occasionally the odd one might need another thump with a drift. I must have saved days of work over the years - and don't mutter "butcher" under your breath, it preserves both case, cover AND screw from damage!
What happens inside the thread is that the upper face of the male thread and the lower face of the female are in close proximity and under great pressure. I was told at the time that the high zinc content die cast ali castings and the electrolytically zinc plated steel screws produced a perfect path for molecular flow from the iron in the screws to the aluminium in the castings, but that's all a bit scientific fur the likes of I. ;o)) Driving the screw into the thread simply shocks the two faces apart and slightly recesses the shoulder of the screw into its hole at the same time.
Regards,
Kim Siddorn