A seal assembly for use with a casing hanger that includes a pair of split rings held together by a threaded fastener. Torqueing the fastener axially compresses one of the rings so that it expands radially inward into sealing engagement with a wall of wellbore casing, and radially outward against an inner wall of a wellhead housing. Support grommets are provided where the fastener enters and exits the compressible ring. Protrusions on a side of the support grommets project into the compressible ring and create a sealing interface between each support grommet and compressible ring. O-rings line inner circumferences of the support grommets to seal between the support grommets and fasteners. A threaded end on a lower end of the fastener has a diameter less than an inner diameter of the O-rings to prevent damaging the O-rings during assembly.
