Abstract: A cartridge heater for insertion within a heater bore-hole or cavity having a peripheral cavity wall surface. The cartridge heater includes an elongated generally cylindrical sheath having a heating element disposed therein and having an internal spring chamber intersected by a pair of elongated slots formed in the sheath. A generally V-shaped spring member is disposed within the spring chamber and has a pair of arcuately shaped arm portions projecting outwardly through the elongated slots for frictional engagement against the wall surface of the cavity to retain the cartridge heater within oversize cavities. The spring chamber may be formed as an extension of the forward or rearward ends of the cartridge sheath.