A preheater assembly of a glowplug and a hollow sheath extending longitudinally from a bottom external zone to a top external zone, the sheath extending radially from an inside surface defining a cavity towards an outside surface, the glowplug being inserted in the cavity, the tip projecting from the sheath through the bottom external zone, the inside surface of the sheath presenting a second thread meshing with a first thread of the glowplug, the inside surface of the sheath presenting a first conical seat bearing against the first conical bearing surface of the glowplug, the outside surface of the bottom external zone having a third thread screwed into a cylinder head and a second bearing surface that bears against the second seat of a cylinder head.