This linear bearing has four tubes that are filled with a number of bbs. The bearing has a cap that is held on with two screws.I have now included the 6mm and 8mm versions of the bearingExternal mounting is performed with zip ties passed around the decorative ribs on the bearing.I designed this as a replacement for the Lm8uu 8mm linear bearing.These bearings are louder than a standard bearing. It is important to note that using steel with copper is not the best solution. Steel shot are a better solution, but not as easy as bbs to get ahold of. besides........These bearings only cost 35 cents to make. Including the copper bbs.Bring Your Own BBs1/4" Circulator version added.6mm which uses 1.77mm copper bbs or steel shot 8mm which uses 1.77mm copper bbs or steel shot 1/4" which uses 1.77mm copper bbs or steel shotYou can use either 1.77 mm bbs or find some 1.77 steel shot and make your own bearing. The bulk of the bearing is due to 3d printer limitations concerning walls.I suggest printing at .2 layer height.For ABS plastic, I suggest that you glue the bearing together with some Acetpaste. (Acetone and melted abs).I prefer PLA for this bearing, as the strength adds to the life of the part.P.S. Out of everything I've built, I really think that this part is the most useful.