90A Durometer Urethane Back-Up Rings -236 Dash Size, Per Pack

Urethane has a natural elasticity that lets you stretch the ring enough to slip it over a shaft or into a groove, and once it's seated it contracts back to its original dimensions. That recovery helps it conform snugly to the mating surfaces without gaps, which is exactly what you need for effective extrusion resistance. The 90A durometer keeps the material firm enough that it holds its shape under pressure rather than deforming and flowing into the clearance gap the way a softer material would.

Sizes line up with O-ring dash sizes, so a -236 back-up ring is meant to correspond to a -236 O-ring size.

Precisely, these 90A durometer urethane back-up rings are designed to provide excellent extrusion resistance, preventing your O-ring from deforming under pressure.

Sizes on these back-up rings correspond to O-ring sizes, so a -236 back-up ring is meant to pair with a -236 O-ring size.

Correct on that. Back-up ring sizes run parallel to o-ring dash sizes, so a -236 back-up ring pairs with a -236 o-ring. No conversion needed, just match the dash number straight across.