I'll try to remember to ask the machinist what sized outer o-rings he uses on the 5.5 ml j-
tank tomorrow. Inner o-rings tend to vary depending whether you've got black, white, or colored caps, & whether the tank's 3 or 5.5ml I could probably give you a definite answer if you have a colored tank, otherwise I might suggest you just take your rings down to the home depot & see if you can find a match.