Don't know why, but building my Prusa i3 (lasercut in nominally 6mm ply), after zip-tying the bearings into the Y-carriage, I found it bumped into, and was pushed up over the y motor at it's far end. Not much, maybe less than a millimetre, but that won't do.
The Y carriage is designed to have the LM8UUs zip-tied in and has holes 20mm apart to put the ties through and a slot to hold the bearing straight.
The other LM8UU bushings (such as the one this is based on) have wider spaced holes than the holes intended for the zip ties, so that's the modification here. I moved the holes closer in, which means fitting is tight, but not impossible, even with the bearing already zipped in.
The bushing gives a rise of a couple of mm, which is enough to allow the carriage to move without crashing into the motor.