You need to lubricate the O-ring with petroleum jelly (vasoline) first. Then, I'm only seeing a diagram on what I have and it appears that the O-ring is installed on the injector first, making sure it has a rubber seal, and a spacer, and then the injector is pushed into the socket with even pressure. There is an "injector holder," (part # 999 5533-8) a special tool that is used to pull the injectors out as well as putting them back in. The "holder" is clamped onto the fuel rail and gives uniformity accross all injectors and insures alignment and equal pressure when pulling them as well as pushing them down, by way of a screwing action of the holder...the screwing action lifts the injector because the injector in the "holder" slides up and down with the screwing of the holder...hard to explain without you seeing the diagram. I'm not sure if this should be attempted without the proper tool. The ends of the injector (needle and nozzle are very fragile and you can break them easily). I can send you the diagram if you will email me.

How did you get the old one out? If you didn't use the "holder" and you insist on doing this job without the holder, then I would look at the others and eyeball how far you need to push the new one in...or if you can get a thin tape measure in beside the injector, I would use it as a guage as to how far you push in the new one. Hope this helped.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.