With proper care and grease (the end piece of the probe should be lubricated) these O-rings remain functional during normal seasonal operations in our latitudes for a very long time. Should they need to be replaced we recommend using a hardness of 70 (NBR or FKM). The three piece O-ring set fits the SUN, UN, and UNG adapters. The two larger O-rings can easily be replaced without a problem using a special tool which you can borrow from ESA. In some cases, the removal of the adapter is necessary to replace the smallest O-ring. For details please refer back to the “service” page on ESAs website.