Nyloc nuts are the staple fastener of the karting world, because they resist loosening from vibration so effectively. That said, they do loose their grip when they get old, or tightened/loosened too many times, so it's worth replacing them regularly, especially because they're so cheap!
These have an 6mm thread, and you tighten them with a 10mm spanner/socket.
SOLD IN PACKS OF FIFTY! If you put a quantity of "1" in your shopping trolley, we'll send you fifty nuts.