Review on 🔩 J&J M3 Brass Insert 100pcs - 5mm OD, 5.7mm Length, Female M3 Thread - Heat Sink or Injection Molding by Mike Bogan

I have had very good results with these as I have tested them in many different hole sizes from 4.2mm to 4.8mm wide. A straight hole 4.2 or 4.3 wide and 7mm deep seemed to work best, another commenter also had success with a 4.2mm hole. However, they are tapered, so a 4.2mm wide, 7mm deep 8 degree tapered hole is a better fit for capping the insert with plastic. In CAD I created a 4.2mm wide hole and beveled it 0.4mm at an 82 degree angle to get an 8 degree angle at about half the depth. This provides good coverage around the insert with minimal overshoot. Hand insertion can be tricky when using straight holes. I have found that straight inserts are made straighter using straight holes and tapered inserts with tapered holes. Although for most it doesn't matter much if you use them for 3D printed parts.