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Backcap Medium (with O-Ring), Torch Head Accessories.
Practical answers on choosing, fitting and looking after the individual parts of a larger gas saver torch set-up.
Usually not. Collets, bodies, insulators, back caps and cups are made to fit one particular torch series, and parts from a different series, or from a standard set-up without a gas lens, often will not thread together or seal properly. Even a part that seems to fit can leave a small gap that lets air into the shielding gas.Match every part to the torch series given in the torch manual or marked on the old part. When several parts need replacing, changing them as a matching group is the safest way to avoid mismatches.
Back cap length is mainly about how much tungsten you need to fit and how much room you have behind the torch head. A long cap takes a full-length electrode, a medium cap is a general-purpose compromise, and a short cap helps in tight spaces but usually means the tungsten has to be cut shorter so that it fits.Caps are made for a specific torch series, so match the series first and then pick the length. Whichever you choose, check that the cap has its O-ring and that it tightens fully without forcing.
The O-ring seals the rear of the torch head so shielding gas cannot leak out and air cannot be drawn in. Heat and repeated tightening and loosening can make it dry out, crack or flatten.Signs include a visible crack, a flattened or pinched ring, or porosity and discolouration in the weld that you cannot trace to gas flow, draughts or cleanliness. Use an O-ring that matches the cap and torch series, check that the groove is clean, and seat the ring so it is not pinched as the cap goes on. If in doubt, fit a new O-ring whenever you replace the back cap.
A larger lens body and cup spread a wider blanket of shielding gas, and a gas lens generally lets the tungsten stick out further while keeping good cover. Makers' technical guidance says larger nozzle diameters give a longer laminar flow, while smaller ones suit tight spaces but raise gas velocity and turbulence.The trade-offs are a bulkier torch head that can block access or your view of the joint, and the need for correctly matched larger parts. Longer stick-out also leaves more tungsten exposed, so pre-flow and post-flow matter. Gas settings depend on the cup, joint and conditions, so follow the torch maker's guidance and test on scrap before welding a real joint.