How do Glow Plugs Seal?

I am trying to make a seal for a glow plug and I am struggling to find clear documentation on how the seal works. I have found that it is a combination of a o-ring and a tight threading. I have been looking at Bosch glow plugs which are all have metric threads which I don't think have a taper, so I am questioning the sealing ability and I can't find any information on where the o-ring would go. This is the glow plug I intend to use: [https://www.boschsparkplugs.net/bosch-0250201039-80006-glow-plug](https://www.boschsparkplugs.net/bosch-0250201039-80006-glow-plug) Any help would be appreciated!