I've just spent a couple of hours trying to find an answer to this "really simple question". I've found loads of advice on clearance hole sizes but next to nothing on this topic. Any help gratefully accepted. For a small project to create collapsible shelving for outside gardening support, I've chosen to make the shelves in aluminium tube. Mainly because I have a load of it already and a "wide wire rack" has many advantages for what I'm doing. I know how I want to make it and this involves flat end bars across the end of the tubes, drilled and then screwed through into the ends of the tubes. The intention is to use self tappers, ideally stainless steel to keep it clean over the years. The head needs to be a flat design such as pan head or even a hex head. I've no issue with clearance holes in the flat bars! What I'm struggling with is how to select the best size of screw for the holes in the tubes. While I understand the basics of how a screw is measured - they are all sold quoting the major diameter (outside of screw thread). Here I need to know the minor diameter (core diameter) to avoid binding of the metal to metal when the screw goes in). If I could find a conversion table I'd maybe be able to guess but I suspect it gets more complex as different screw types, different threads and/or different manufacturers will have different ratios of major to minor diameters. So, specifically I have 13mm aluminium tube, with 3.5mm walls. I should therefore have a 6mm hole. Having measured a few of the tubes it's actually slightly smaller. The range I've found is 5.85 - 5.95mm. What size self tapper do I need to buy to ensure that the screw goes in without binding but makes a good tight fit? ...and yes I could buy a few packets and try them all but I've already got thousands, new in box through recycled in unsorted containers etc. so I want to minimise unneeded purchases if I can - it's cheaper too! I suspect that 8mm is too large. 6mm is going to be too small, so I'm guessing it will be a 7mm screw but I don't know and as that's not going to be as easy to get I'd like to know how to size it properly.