The factory gasket is made from metal but the design is the bead type which is not as effective in sealing as the super grommet design. Most aftermarket metal gaskets are of the grommet design which is better, but the TOMEI Super Grommet design gives you the highest sealing efficiency possible and increased durability.
With each model change from Subaru engineers the improved engine design from the GC8 to GDB and A models had the increased the rib of the outer bore diameter which improved the gas sealing characteristics. But the results of that change made the water and oil galleries seal decrease. So when the power is increased there is also the increased possibility for oil and water leaks to occur. TOMEI engineers had a second look at the design of the rib height and the Super Grommet Design and they have achieved an overall improvement in overall seal.
They have made multiple thicknesses available to achieve your desired compression ratio for your application. Now you can build an engine with higher accuracy to suit your needs.