You didn't mention which engine this was for. Keep in mind especially if this is a new part, Cox offered two different thickness circlips. If your re using the existing, no worries. I used a thicker circlip which wasn't compatible with my tank. As a result, the circlip came out and almost exited through the side of the case. I still use this engine and while it destroyed the piston and cylinder, the case has a large bulge on the side of if. In addition, certain reeds like a Teflon reed is also thicker which requires the thinner circlip. Teflon reeds also like to stick to each other and it's difficult to separate them. I prefer the Mylar or the stainless reeds but the old copper star reed works quite well but is easily damaged. Always test the reed after installation with a screwdriver to insure it moves. Next hook a syringe with tubing to the back of the tank venturi and try to pull the plunger out. If you can pull the plunger at all the reed isn't sealing. Remove it, flip it over and try again. Use a drop of oil on it when doing so. If you can't achieve a leak free test try another reed.