Thread
More to the point is that the dark bottom cheesehead Torx screws have a metal thread to mate with holes in the metal chassis, and the top 'silver' cheesehead screws have a coarse thread to mate with the corresponding holes in the bezel without ripping the thread out of the ABS plastic when they are overtightened. And to restate the already well-known, all of the screws have recesses in their heads to mate with a Torx-15 driver (splined cylindrical tip) that has at least an 8-in shaft. The driver doesn't need to have a central (axial) hole in the tip because the screws that Apple used are not security screws (Resistorx or Pin-Torx).
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Thread spec by measuring a sample:
Size #8 USA standard self tapping screw
Standard USAS B18.6.4-1966
Type AB:
18 threads per inch, right hand thread
Major/minor diameters .163 OD/.119 root (inches)
standard 60 degree thread form (there exist designed for plastic custom metal screws with a more acute angle but these are not that kind)
Point type B
Nominal length of shank: .625 inches
If you inadvertently substitute the common Type A self tapping screw with nearly the same major and minor diameters but 15 threads per inch some serious cross threading and thread destruction in the housing could result .
Size #8 USA standard self tapping screw
Standard USAS B18.6.4-1966
Type AB:
18 threads per inch, right hand thread
Major/minor diameters .163 OD/.119 root (inches)
standard 60 degree thread form (there exist designed for plastic custom metal screws with a more acute angle but these are not that kind)
Point type B
Nominal length of shank: .625 inches
If you inadvertently substitute the common Type A self tapping screw with nearly the same major and minor diameters but 15 threads per inch some serious cross threading and thread destruction in the housing could result .
I'm sure if worse came to worse you could find non-Torx screws to fit those specifications.
Also, if you are taking apart Macs, be careful not to confuse the Torx screws that hold the chassis and CRT in the computer with the case screws. Some later model Classic IIs actually had gold-colored screws for the chassis and CRT to prevent confusion. The silver screws look nearly identical except the chassis/CRT ones are shorter. If you use these in the case it won't fit together, and if you use the case ones for the CRT, you'll get what resembles a huge pimple on the other end of the case...
Also, if you are taking apart Macs, be careful not to confuse the Torx screws that hold the chassis and CRT in the computer with the case screws. Some later model Classic IIs actually had gold-colored screws for the chassis and CRT to prevent confusion. The silver screws look nearly identical except the chassis/CRT ones are shorter. If you use these in the case it won't fit together, and if you use the case ones for the CRT, you'll get what resembles a huge pimple on the other end of the case...