RB20/25/26 Head dowel
These dowel pins often go missing when working on the engine. They are absolutely necessary to ensure the head, block, and gasket are aligned perfectly when they are torqued down. If you decide to take your chances and assemble without, you may find yourself with a blown head gasket, or even damaged pistons and/or block when they collide.
Priced individually, you need TWO for an engine.