TW4 1/4" Bore High Load Standard type Top Ball inch type linear motion ball bushings/bearings, inner diameter(bore) is 1/4" inch dimensions with built-in seals, the Topball slide bush is a high performance bushing with three times the load capacity, capable of providing up to 27 times normal travel life of a conventional slide bushing, high accuracy grade linear bushings, series mainly used in the USA, NB brand (Nippon Bearing Linear Systems), made in Japan.