Steve, I don't know who you got your kit from but, the ones I get from Rick have a replacement valve stem assembly included. He sells the same kit for the 160 and 180, the stem has to be shortened a bit if going in a 180. Your first CO2 gun, you say? You're going to love it. I got my first one about eight years ago, have a whole slew of them now. It's nice to just shoot without all the pumping.
And so it begins! Co2 opens up a whole nuther arena for collecting and tinkering, some very cool stuff. Maybe a Giffard in your future?