Casters are great. Garage. I couldn't find the bolt size in the description. Bolts are 5/16 bolts - 18 with 1" thread length. This is suitable for all applications where a metal support frame is present. In my case I have a wooden frame that I needed to thread them through. I used a 5/16 screw to attach the casters to the base frame. The casters have a minimal ground contact area for easy manoeuvrability. They're on the high end of what I was willing to pay for a 4 inch roller, but they were worth it. I have no complaints or regrets. I didn't expect screws to work for my application so it wasn't a problem. the locking mechanism is also smooth and requires no more force than expected. I am happy with these reels and would buy them again for other similar purposes.