(IBC = Inner Bolt Circle, round seal without screw holes)
The centering of the seal is accomplished by the outer diameter of the gasket in combination with the nominal screw diameter,keeping the gasket (ring) in place. Please note that a higher surface pressure can be applied to the seal by using IBC form using the same bolt-torque values. Since the surface pressure has a decisive influence on the sealing-class, an IBC seal is generally the more advantageous option.

Dimensions of Gaskets Form IBC Raised-face and flat-face flanges According to EN 1514-1 Form IBC