If thermocouple is from ordinary tank-type water heater, then it is a generic part with different lengths 15" 17" 21" 24" 30" available at hardware of home center.
I suggest reading manual for installation information, advisories, and required maintenance.http://waterheatertimer.org/pdf/Rheem-Ruud-water-heater-parts-guide.pdfhttp://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
There are good utube videos on replacing thermocouple.
Tip of thermocouple has to be positioned in the pilot flame.
The heat on tip of thermocouple causes millivolts of electric current that travel back to the gas control valve and the electrical signal causes gas control valve to keep gas flowing down the pilot tube to the pilot light.
If pilot light blows out or if pilot tube is clogged and needs to be cleaned, or if gas valve has gone bad, then tip of thermocouple cools down and electric signal back to gas control valve causes gas control valve to shut off gas going to pilot.
When attaching thermocouple to gas control valve, it is finger tightened.
Be careful and do not cross-thread thermocouple. Just use your fingers to start the nut.
Finger-tighten thermocouple and then add 1/4 turn using small wrench.
The end of thermocouple has a small piece of insulation that isolates the electric current from body of gas control valve. If thermocouple is overtightened, the insulation is crushed, and electrical current millivolts short out and no signal reaches gas control valve, so gas control valve keeps gas turned off to pilot tube.
If this is a tankless water heater, then open following link with troubleshooting resources and contact information for Rheem.http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
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