The ones I buy come with M5 button heads, but you can tap and drill whatever works. An M5 or 10-32 drill and tap set would do the trick. Make sure you use a "just right" length screw. Too short, obviously you won't catch enough threads, too long and you could hit a sealed bottom bracket.
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If someone tells you that you have enough bicycles and you don't need any more, stop talking to them. You don't need that kind of negativity in your life.