Seals, regardless of their principle, are usually preloaded so that their contact surface is pressed towards the surface it seals against, even before there is a pressure drop over the seal. In conventional seals made from polymeric materials, one normally achieves the pre-loading or pre-stress by compression of the cross section. To be able to counteract the materials predisposition for settlement, the cross section has to be considerable bigger than for piston rings in metal. Despite this, seals in polymeric materials will lose their pre-stress and tightness during service and this becomes a determining weakness for the sealtype. Metal split rings will have the pre-stress introduced by the curvature stress in their high and narrow cross section. Because of the normally small form errors found in the cylinder bore, these piston rings do not achieve full contact around the entire cylinder bore. The rings will just touch on a number of "high spots". Before a pressure drop over the seal has developed, this functional principle lacks the power that can bend the seal and generate full contact between the seal and the cylinder bore. In the Metseals piston ring one creates the surface contact between the seal and cylinder bore by applying high compressive stress in peripheral direction. This will force out the thin and flexible seal so that it has, and only can have, a good and even surface contact with regular contact pressure. The relatively high-pressure stress on the periphery is due to the fact that the circumference of the sealing ring, before fitting, always is slightly bigger than the biggest allowable circumference for the cylinder bore. For every tolerance on the cylinder bore, H8, H10 etc., and their maximum diameter, there is a seal suited for the actual tolerance. The seal is selected to ensure that the seal’s outside diameter exceeds the maximum permitted bore diameter by a specified minimum dimension. Thus, the pre-stress is achieved. As a rule the same diameter difference and pre-stress is chosen for all different cylinder bore tolerances. The maximum contact pressure will naturally increase with increased tolerance and difference in diameter. A seal suited for H8 will produce a maximum contact pressure that is only 40 % of corresponding pressure for an H10 tolerance. The pre-stress will arise when the piston seal and piston is fitted in the cylinder bore. As it is principally only the piston ring in metal that is responsible for the pre-stress, a Metseals piston seal is free from settlement. The O-ring can be allowed to have considerable settlement, otherwise the capability to seal between piston ring and sealing groove in the piston is jeopardized. The magnitude of the contact pressure that is developed by the pre-stress is normally about 25-50 bar, both for Metseal and newly installed seals in polymeric materials.