When fitting a 4mm brass sleeve to a 3mm motor shaft, it's good to know that brass tube comes in varying thicknesses. All my 4mm brass tube was thin walled and made a sloppy fit. It was only after I discovered 4mm tube with 0.45mm wall thickness that I was able to get the perfect fit. The sleeve is locked into place by allowing solder to flow into the gap formed by the D section motor shaft.