In a Racor filter, an adaptor is provided to threadably mount a water collecting bowl to a can containing a filter element. In order to prevent the adaptor from rotating with respect to the can, the adaptor has an axially extending rim with a non-circular cross section that complements a non-circular opening through the bottom of the can. In a preferred embodiment, the non-circular opening has two chord segments disposed opposite from one another which engaged two chord segment flats on the threaded rim.