Back-Up Rings eliminate tendency to extrude into clearance at high pressures. They operate under slight amount of squeeze. Back-up rings are made of metal and manufactured from the stronger plastics. This provides sufficient elasticity to permit stretching over the major diameter of a piston and then snapping back into the gland groove cut into the piston. Back-up ring is a high shear modulus material placed in a seal groove between the seal & the clearance gap to protect the O-ring and prevent extrusion. Back-up rings are washer-like devices used to prevent seals from extruding through gaps while under pressure. Back-up rings are used to improve performance and extend the life of O-rings by supporting them in higher working pressures. They are also used to prevent O-ring extrusion due to high systemic pressure or loose fitting parts.