Back caps typically come in 3 sizes (related to their length):
Long (default size).
A new Tungsten electrode is normally about 7″ (175mm) long. As the Tungsten electrode gets shorter, you can change the back cap to a smaller size. A smaller back cap allows the TIG torch to be used where space is restricted. So, keep some shorter Tungsten electrodes back for this purpose, rather than using the electrode up completely.
Make sure the back cap fits your particular type and size of TIG torch before purchasing.
TIG Back Cap O’Rings:
Back cap o-rings prevent the escape of shielding gas from the back of the TIG gun head. More detail on back cap o’rings (such as size and type) can be found in the article ‘TIG O’Rings‘.
Occasionally, there is some discussion about lubricating the o’rings with silicone grease to help prevent damage to the o’rings and extend their life, (silicone grease doesn’t damage the o-ring material).
If you decide to use some silicone grease on the back cap o’rings, you need to take care, as the silicone grease could cause weld contamination, especially as the TIG torch heats up and the silicone becomes more fluid.