The OEM Subaru EJ257 2004-2007 2.5L STI head gasket is multi-layered steel. Subaru 2.5L engines built on 06/2006 and before require a different part number than 2.5L engines built on 07/2006 and after.
Gaskets sold individually.
02-07 WRX/STI stock head gaskets are .6mm (.023 in) thick.
Please refer to your drivers side door jam to determine the build date of your vehicle.