Review on Yasorn Expansion Screws Stainless Ceiling by Brian Kumar

An 8mm dowel requires a 12mm hole in the concrete to hold securely. It's just over 7/16. Not having a 12mm hammer drill, I used a 7/16 bit to drill through the concrete and "wiggled" it a bit as I drilled to create a slightly larger hole. Care must be taken when doing this, however, as too much "wiggling" will cause the hole to enlarge slightly, reducing grip strength. If you want stainless steel and need to be able to take what you've secured off without a bolt sticking out to trip over, this will work well. Suggest you buy a metric impact drill and use an impact drill.