There is no o-ring on the threaded housing. Fuel should only travel through the plastic monitor, not around it. If you are re-using the plastic monitors, try using a new one. The leak may be due to a worn-out plastic monitor. If not, then replace the 2 o-rings INSIDE the housing. GTP-2200-06V on top. GTP-2200-12V on the bottom. This does not apply to the GTP-3318 high pressure monitor holder, which is rarely used except on high pressure pipeline use. It is all stainless, not gold like our standard MiniMonitor “Millipore”-type tester.