Formula track axle lock nuts
Because most people usually overtighten their axle nuts like uncivilized gorillas, these little bastards break all the time on Formula hubs. Replace 'em and keep a few extras on hand just in case human restraint ain't your thing.
These will work on other track hubs too but you may need to get creative with some axle spacers.
Sold in pairs.
• thickness: 4.7mm
• choose front or rear (9mm or 10mm)