These washers are for M6 rack screws that are pretty standard. They are quite a big diameter and spread the load well. The screws locate tight in the centre and don't move about so they are concentric when you tighten them up. Most importantly they won't mark your gear.
As they are quite a big diameter and slightly more so than the cheaper softer ones so you need to be careful when using on the very top or bottom of racks as it can catch on the racking. It could also overlap units when using 1U or other rack units where the holes are at the very top or bottom. There's a way around this should you come across these problems and that's by shaving a slice off. It worked for me and they still perform the job well.
Unlike the cheap flexible rubber washers that come with rack cases - these ones are rigid and do not distort. So the rack screws are held more firmly, with little tendency to slip sideways. In practical terms, I don't know if it makes a difference, but the units fixed in my rack case definitely feel like they are fixed more robustly...
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