I purchased casters to replace the wheels on the wire cart I use to store my computer/monitor. These were the only 6mm stem unicycles I could find. The twin wheels that come with everything tend to pick up hair and make the wheels pop off. (yes, I suck). You can see the nut in the photo, unfortunately mine was pushed deeper into the wheel making it impossible to tighten with a wrench. The stem is too short, it needs to be at least 1/2 inch longer to hold the legs well. The wheels were wonderful for about an hour and rolled very well until they bent out of their sockets and fell off. I'm buying a new car with big 3 inch wheels. It wasn't worth wasting time and effort to fix the old one.