Abstract: A seal for a fluid jet apparatus includes an assembly with a head portion and a stem portion. A body has a bore formed therein, the bore has an upstream portion of a first diameter, a downstream portion of a second diameter which is less than the first diameter and an axially extending converging portion which connects upstream and downstream portions. The stem portion has a lesser dimension that said second dimension. The stem portion is disposed within the downstream portion and the head portion is disposed within the upstream portion. The upstream portion extends for a considerable distance upstream of the head portion. A maximum cross section of the head portion is less than the first diameter and greater than the second diameter, and a surface to surface seal is formed between the head portion and the axially extending converging portion. This seal may be applied to either a nozzle or a swivel of a fluid jet assembly.