The 3/4 in. x 3/4 in. Female Adapter FPT has one end that is threaded to fit on a 3/4 in. filter piece. The opposite end of the Adapter FPT is narrow and is meant to have a 3/4 in. tube slide over it and fit tightly. If a tighter seal is wanted then the use of hose clamps or ratchet clips would be advised.