For JG™ sliding doors and windows there are three replacement parts: two adjustable carriage and acetal roller units, one with a plastic body and one in zinc-plated steel, plus a keyed window catch and spring. The same roller offered in two body materials is the useful detail. The plastic body is the like-for-like swap, and the zinc-plated steel version is the heavier option where a door has already chewed through a plastic one. Carriages and catch springs are what fail on this hardware; the frames outlast both. Before ordering, remove the old carriage and compare the body shape and screw fixing against the product images, and check whether your window catch takes a key, since a keyed catch and a non-keyed catch are not interchangeable.