PTFE tape for making hydraulic cylinder guide rings to size. It is cut to the length of the circumference and fitted in the piston or cylinder head groove, like a guide ring. PTFE has low friction.
Sizes read width × thickness, in mm, from 3.8x1.5 to 35x4. The most frequent thicknesses are 1.5, 2 and 2.5 mm.
To choose, measure the groove width and the thickness the tape has to fill: half the difference between the bore (or rod) diameter and the groove diameter. When cutting, leave a small gap between the ends.