This kit definitely needs a simple install diagram. It is not complicated, but may not be readily apparent that the bigger round o-ring squishes into the oval hole. That o-ring is 5.5mmx1.5mm. This size is used elsewhere on the Impact. The other tiny o-ring seals the atmosphere hole for 2nd reg, which technically you shouldn't need a o-ring for, but by sealing that junction it preserves troubleshooting functionality of the hole on the surface. A nice touch by FX.
You should use a new x-ring seal under the gauge to ensure best seal possible. Also pay attention to the threads on your gauge if you are moving the one from the factory location. On mine, I had hard clear sealant, and this prevented the gauge from threading in far enough into the angled bracket to achieve seal. If your gauge abruptly stops when you thread it in, this is probably what is happening. Resistance should start ramping up smoothly if the gauge is engaging the seal at the bottom of the hole.
No leaks in mine after putting in a fresh x-ring seal under a gauge with clean threads.
Edit: I just re-read and realized you were really asking about the grub screw only. Mine had nothing under it and the FX parts diagram doesn't list any seal. I have the same question about how this screw is supposed to seal.
