slide into place with almost no effort, and I can actually take it back out again from the top with almost no effort. You must use enough vaseline on the O-rings though, on both of them, otherwise it won't be easy to install the injector, and due to how sticky the rubber O-rings are, pinching or tearing them is easy, with enough lubrication you shouldn't have any problems. Try replacing both O-rings on that injector and try again with vaseline on both the top and the bottom this time.