Some metric users are finding it difficult to achieve the .2mm to .5mm gaps that are required for a "sliding fit". In most cases, metric sizes offer either 0mm or 1mm gaps which are either too tight or too loose. Some builders have taken to using a lathe to reduce insert diameter to produce a .2mm gap.

In the pic, tubing has been converted from Imperial to metric to show the acceptable gap range.

.750in. tubing = 19mm
.625in. tubing = 15.9mm

.049wall = 1.25mm
.058wall = 1.47mm