These are the same machined aluminum injector seats used in Radium fuel rail kits. They are required when converting an OEM side feed injector to a top feed injector that uses a 14mm lower O-ring. They simply "pop" into the intake manifold (1 required per injector). They seal to the machined injector counterbore using the included FKM O-ring. The 2 sizes are based on the intake manifold's inside diameter of each injector counterbore. Below are just a few examples of some popular Nissan engines.
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