This plug is designed specifically for M.T. axial Ø25 boring bar holder (part n. NKM0112560) but i guess it can be adapted to most boring bar holder, i uploaded both the stl and the .f3d files if you need to modify it.

It replaces these plugs and allows internal coolant flow in boring bars and drills, improving tools performance, durability and chip evacuation.

Print settings

I tried them both in PETG and ABS, i've been using them for about a month now and they still work perfectly.

I suggest using a high upper shell layer and wall number. I printed them with 6 walls and 6 upper shell layers and 20% infill, but they are so small that it may be even worth to print them with 100% infill just to be sure.