Abstract: An easily maintainable interface for magnet dewars which includes a low h leakage disconnect fitting that retains the vacuum integrity of both the dewar and the transfer line is provided. The fitting includes a bayonet-type insert having separate coils disposed about a central conduit for cooling or heating the unit depending upon intended use. Liquid helium is introduced axially through the bayonet which is adjustable to a purge position wherein either gaseous or liquid helium may be introduced to clean, heat or cool the unit, or a flow position wherein liquid helium is passed through the bayonet into the supply line.