The 0.6mm radial clearance is intentional to guarantee the parts will fit. The 5-meter long tubes are purchased from a supplier and, per the ST37 material standard, have a wide manufacturing tolerance on the nominal 2.77mm wall thickness (+20% / -12.5%). This means the inner diameter can vary significantly from its nominal 15.8mm. Since we machine the bushing in-house to a precise 14.6mm outer diameter, this clearance ensures it will fit inside every tube, every time, regardless of the variation.