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Yea seems this is a common issue with 3D printers, it is just plain physics that the printer will drag the filament across the hole and you end up with undersized internals. The answers seem to be over sizing any internal bores or using a slicer that adds many thousands more triangles. I had the same issue so I just purchased an elcheapo chinese step drill that ended at 25mm (heaps on ebay) and used that to bore through my internal tube fittings. I think if you printed with 2 or 3 shells it would not be a problem to do this, if anyone else has a good answer for printing accurate internal holes I'd love to hear about it.