These are the same machined aluminum injector seats used in Radium fuel rails for Mazda/Eunos Cosmo and RX-7 rotary engines. They are required when converting the OEM 16mm injector seat's counterbore to 14mm which is more common with popular injector lower O-rings. They simply "pop" into the OEM injectors bore. They were designed specifically to be compatible with or without the OEM fuel injector diffusers installed. They seal to the machined injector counterbore using the included Viton O-ring.