In soft polymeric seals, extrusion can occur at high pressure, provided the seals do not have special protection rings for extrusion, which are made of proportionately hard material. Another way to avoid extrusion is to limit the clearance gap to about 0,05 to 0,3 mm. When using Metseals metal ring there is no risk for extrusion. The gap have a standard value of 0,25 mm, just to permit a small wear of the piston guiding ring, but in the first hand to produce a twisting power on the seal. This will reallocate the contact pressure over the seal so it will decrease on the low- pressure side and increase on the high-pressure side. One will then get a relatively even and low contact pressure over the total width of the seal.