Injector

[FullPEAK]

Hey guys quick question what size O ring with fit tight n snug on the bottom of my injector pictured on the right ? The blue is my stock which already has the grommet/fitting on . I can't use it on the green injector.
Thanks

 

[FullPEAK]

FYI I believe these are RC 550 injectors if that helps. Please correct me if I'm wrong.