Water Heaters - Hot Water Heater Thermocoupler - Too Long OK?

The pilot light won't stay lit on my 40 gal Rheem natural gas water heater.

It had a thermocoupler in it that was 18" long.

I tried replacing it with a 36" long one that was available at Lowes last night but it didn't solve the pilot light going out problem.

Could the problem be the 36" thermocoupler doesn't work in a heater that originally had an 18" one? Or shouldn't the size difference matter?

The length of the thermocouple makes no difference, just neatly coil up the excess out of the way.

Make sure the end of the thermocouple is touching the pilot flame and be sure the other end is securely screwed into the valve (do not overtighten).

I assume the unit works for a while and then goes out at some later time. While a bad gas control valve could cause this, it is rare for these valves fail. You must check everything else before condemning the control. You can begin with a cleaning of the pilot tube and assembly. Also look for any flue restrictions or down drafts. Sometimes, if the heater is located in a hot location, such as an attic, you can experience this problem. Others have offered additional sound advice on this type of problem in previous threads.

If, on the other hand you have a good thermocouple and the pilot goes out as soon as you release the "Button", then your control valve is the culprit.

If I were to move the end of the thermocoupler away from the pilot light to the outside of the heater & put the flame of a cigarette lighter right on it, would that eliminate the thermocoupler being the problem? By doing that, the pilot light should stay lit, right?

The cigarette lighter flame may be hotter or larger than the pilot flame. If this is the case it may work just fine in your test conditions. Thermocouples tend to become weak, a hotter flame may enable it to put out just a bit more voltage.