The long hole is to give the drill clearance and allow for several sizes. It workes for me from drills 3mm to 8mm. These are the ones I use most of the time. The other hole is dimensioned with 3.3mm - core hole for M4 thread. You have to do the threading by hand. With PLA that worked great for me. I currently print the part with PETG and want to try that with that kind of material also.