Abstract: Disclosed is a substantially tubular device reversibly applicable to a knockbox bar for espresso coffee machines to cover the bar. The device is elastically deformable and is applicable to a knockbox bar by spreading a slot extending longitudinally into the device over the entire length thereof. The device protects the knockbox bar on which it is applied from both the impact against the filter holder assembly and the coffee powder ejected therefrom. The device includes, close to a first end, flaps arranged longitudinally allowing easy cleaning of the filter edge from any powder possibly compacted therein. The device further includes, close to a second end opposite the first, grooves acting as facilitated cutting lines and by virtue of which it is possible to shorten, if required, the device so as to adapt it to the length of the knockbox bar to which it is be applied.