It's been a long time for me. Is this an Allen or is it a 10MM hex. I remember that it was a challenge when I replaced my expansion valve, but I don't remember how I did it. I have a very thin wall, short 10MM box end that I might have used. I do remember that it is one that will try your patience 1/12th of a turn at a time.

Sorry I can't help. It's one of those things you just work through.