Thunderball,
The rib screws are on pretty tight, but not impossibly so. If you've got a interchangeable-bit screwdriver, it sometimes helps to insert the appropriate bit without the driver, and give it a couple of raps with a small hammer before attempting to loosen them.
I have no idea if there's any set amount of torque for the rib screws. It really shouldn't make any difference, as long as the rib stays on firmly.
