Wow.... umm.... yeah. I'm sorry you're having so much trouble too. I'll dig out my spare fuel rail and FPR and make a video this weekend if need be... you are making this too hard or something is F@$%ED up.
No, you can't install the O-ring before the FPR. A thicker O-ring could cause problems if it is too thick. Get the right size... it'll be metric. 10mmID x 2mm is what I come up with measuring mine and accounting for changes in shape from prior service.
Step one: With FPR off of fuel rail unthread lock nut all the way with just your fingers (this means the lock nut will move UP toward the FPR's body).
Step two: screw FPR onto the fuel rail by hand until it stops... DO NOT TIGHTEN ANYTHING!
Step three: Unscrew the FPR just enough to get it pointing the right direction.
Step four: tighten lock nut.
NOTE: To help ensure that the o-ring isn't getting pinched you may wet it with a little soapy water before threading the FPR onto the rail.
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