An electrical sleeve heater for heating an elongated part has a metallic and tubular casing extending coaxially along an axis with the part and surrounding the part. The casing has a front end is with a radially inwardly open cutout. A electrical heating element set in the casing is energizeable to heat the casing and the part surrounded by the casing. A thermocouple has a sensing tip in the cutout. A retaining body of at most low thermal conductivity is fitted in the cutout, holds the tip out of contact with the metallic casing, and urges the tip radially inward into direct engagement with an outer surface of the part. The retaining body is a molded part, a plate, or a spring of a material with low or negligible thermal conductivity.