In case you want to replace the plastic arms on your copter frame with 16mm carbon fiber or aluminum tubes. Fits my "whirlwind FY450" clone frame from HK, probably also others but I am not sure about the hole pattern of those. Because the holes of the upper and lower frame piece are not exactly in line, the two holes are tilted so that you can still use 3mm bolt through - holds the arm just fine only with the two bolts, I also use ziptie throuht the two central holes (see image). Added version 2 - in case you have frame which uses 40mm brass standofs for battery tray, or you just want to have more space between the upper and lower frame pieces. I printed in ABS, two loops and 50% infill. I guess you could use less infill, the real stress would be held by the bolts. I had to drill the holes to size with 3mm drill. I printed v2 with 25% infill which worked just fine, probably could go even lower.