I have a couple of dumbbells at my home gym and the star nuts holding the weights were getting loose. I tried to find the exact measurements of these nuts on the net so that I could model or buy some extra parts, but I couldn't find it anywhere. So I grabbed a caliper and just measured the thing, then gone through several iterations (8) until I got a part that fitted exactly on my dumbbell rods.Although I could not get the measurements online, from pictures it seems that most if not all dumbbells use the same screw sizes - even the pitch and the spiral direction seems to match. So I think it's highly likely that this design will fit on other dumbbell brands.I have used the public domain library polyscrewthread, which I include here. Just put it in the same directory and it should work.I have made two versions. The first one is the final design which I printed and fitted perfectly on my dumbbell. However, I have a FDM printer which "overextrudes" about 0.2mm outward in the horizontal plane due to the 0.4mm nozzle. So, if you use a non-FDM printer which does not everextrudes, or you use XY compensation in your slicer, or you somehow manage to get exact measures with what you print, I also made an "-exact" version which has "exact" measurements for the nut to work. So, if you print the nut and it is too loose, you might try this one which has a slightly smaller hole.Also, if you are reading this and you know where to find the measurements information that I couldn't find, I'd gladly thank you for telling. :)RECOMMENDED SETTINGS: print with no support, 0.2mm. Use a medium to large wall thickness and infill for improved resistance, specially if printing with ABS. I recommend at least 5 walls (top/bottom also) and 30% infill. If you print in polycarbonate you can use a little less.