Length matters for access. Short back caps let the torch fit into confined spaces and are used for work inside tubes, tanks and structures. Long back caps hold more tungsten and are used for standard bench work where access is not restricted. Medium lengths are the general-purpose choice.
Back caps are series-specific — a 9 series cap will not fit a 17 or 26 series torch. The thread and the sealing surface differ between series, and using the wrong cap gives a gas leak or a cap that will not seat.
O-rings inside the back cap seal the gas path and degrade with heat and with repeated removal. A leaking back cap shows up as poor gas coverage or as gas escaping continuously. Replacing the cap or its O-ring is a quick fix for a problem that otherwise looks like a gas supply fault.
