Flange gaskets are also referred to as full face gaskets and contain bolt holes on the face of the gasket. If you use this type of gasket then the PCD method will help you measure your gasket correctly. The PCD is the measurement between the centres of opposite holes. This can be measured with a tape measure from the inside of one hole to the outside of the other (as per the image above), providing the hole diameters are equal. It would also be useful to make note of the gasket thickness to ensure you know the complete dimensions required.