[zonker]

Aluminum would require a machine screw and a tapped hole w/ a helicoil insert of some sort.

It was on steel wheels mostly, with the pointed end going right into the sidewall bead. the small piercing on the bead causes no inflation issues with the tire, note i said small piercing... do not try to use a 2" long metal screw to secure the cap lol.

Actually, running screws through the rim into the sidewall was also done as a racers trick to keep the tire from spinning on the rim under heavy acceleration like in drag racing.

Here's the installation instruction sheet that is given with the moon discs from the speedwaymotors website:

INSTALLING MOON STYLE WHEEL DISCS

1. Wheel discs can be applied with tires on or off the vehicle.
2. Place disc on wheel and line up hole with valve stem.
3.Mark back of wheel disc with this location of the valve stem. This will help with placement and alignment of disc when installed.
4. Scribe the location of all three mounting holes, center punch and drill each hole with a #17 drill bit.
5. Warning! Do not drill through the tire - only drill 3/16" deep; mark drill bit with a piece of tape for proper depth.
6. Mount each wheel disc using the self tapping screws. If the hole is too tight, increase the size of the hole with a #16 drill bit.
7. Use blue silicone sealer or loctite to prevent the screws from backing out.