thermocouple extension

Looking for advice on extending the thermocouple lead on a 300*F max remote thermometer. Thermocouple lead was cut three inches from unit. I need 8 feet to source. McMaster has a "thermocouple extension wire " rated at 220*F. I assume this is the insulation & not the wire. If so I could mount it so the sender alone is at the heater design max of 250*F. This will be used in a car engine bay to read temperature of diesel injection line heaters. No politicians were saved in the making of my movie.

Your "rating" is correct - it's the thermal insulation of the cable. You should be able to extend the cable without too much worry.... I've forgotten the mV and current/impedence of the various thermocouples, but the currents are so small that cable resistance shouldn't be a problem, nor cable impedance (ie, capaciatance etc) since it's DC - not audio or video frequencies (ie - no 50-Ohm coax etc). The only issue might be picking up noise, but again - being dc - that shouldn't be an issue. Otherwise a small cap across the meter input should remove the noise (say 0.01uF or smaller?).    I have done similar in my car. A DMM with type-K thermocouple & 1000C leads. The thermoC is near the radiator; its leads just get through firewall. I have panel-mount banana sockets that join the thermoC's banana plugs to the normal DMM probes (separate 2' unshielded wires). Neither noise nor accuracy have been a problem despite the agricultural wires above my feet.....