Back-up rings are typically used in dynamic sealing applications where hydraulic pressure exceeds 1500 psi. The use of a back-up ring extends the life of the O-Ring and thus prevents premature O-Ring failure in high pressure operating environments. Included in our kit No. 293 x 36B is a total of 293 back-up rings in 36 different sizes: -005 thru -018, -110 thru -121, and -210 thru -219.