Abstract: A temperature warning device for indicating when a threshold temperature has been exceeded includes a threaded bolt having a bolt head and a bolt body. The bolt head and bolt body have a longitudinal bore defined by a surrounding surface with the bore having a closed end and an open end at the bolt head. A plunger in the bore has a plunger head and a plunger body terminating at the open end of the bore. A retaining cap is secured in the bore of the bolt head and has a cap hole through which the plunger body freely extends. A sleeve of a fusible material is contained in the bore which melts at the threshold temperature. This sleeve substantially fills a volume of the bore radially between the plunger body and the surrounding surface of the bore and longitudinally between the plunger head and the cap. A compressed spring is located in the bore between the closed end and the plunger head.