An end cap assembly for a fluid cylinder includes a cylindrical casing having a longitudinal axis and an open end, and an end cap closing the open end of the casing. The end cap has a first connecting device for connecting with the casing, and the casing has a second connecting device for connecting with the end cap, whereby the first and second connecting devices interconnect with each other. A backup device is disposed separate from and engaged against the casing or end cap, and prevents deflection of the engaged casing or end cap in a direction transverse to the longitudinal axis.