@ChpBstrd do you know whether there are any water heater experts on this forum that would be interested in geeking out on this with me?
I do not know of any forum WH experts, but I do understand how using the wrong parts on a thermocouple could be a safety concern.
The thermocouple's job is to convert heat (from the pilot light) into electricity that holds open a magnetic valve which allows gas to flow to the pilot light. This way if the pilot light goes out, you're not just spraying gas into the room. If for whatever reason it is possible for gas to go to the pilot light when the pilot light is not lit, then you'd have a gas leak and explosion hazard.
Maybe the thermocouple you substituted does this job fine or maybe not. One way to check is to blow out the pilot light and then see if you can relight it without going through the normal procedure of holding down the valve button. If you can complete this test three or four times, and the pilot light is impossible to re-light, then the thermocouple is correctly closing off the gas supply to the pilot when the fire goes out.
The opposite concern is that the bigger thermocouple might be only barely getting hot enough to keep the gas flow going. Maybe in very cold weather, it would close off gas supply to the lit pilot light and you'd find yourself without a hot shower and unable to relight the pilot. This situation could obviously get annoying, so you may want to take the model number and the old part to your local plumbing supply house (not big box store, the warehouse plumbers go to), and see if they sell the original part. Could be a $25 fix to get it right.
Overall, we have to ask ourselves how much effort and money a 17 year old water heater is worth. You could work very hard or spend money on a rare part only for it to leak a few months later. A major concern is that I cannot 100% tell from the picture whether it has a leak pan installed underneath. If you do not have a pan to catch the leak, the potential failure of this water heater means water damage to your house, which will far exceed whatever depreciated value you might enjoy from milking another year out of this unit.
I've gutted a bathroom to the studs due to a leaking water heater (that was about the age of yours) that was not installed on a pan and it is regrettable. Factories and other industrial facilities have a concept of preventative maintenance, where they will replace working equipment on a timeframe, rather than waiting for items to fail at some random time as they approach being worn out. This practice may seem inefficient, because they're not getting the full lifespan out of equipment. However, in the big picture it is the most economical solution, because they're not shutting down the plant with hundreds of idle workers over and over again as each of the tens of thousands of pieces of equipment break on their own schedules, because they reduce time wasted on diagnostics which is spent to find the cause of each preventable problem, and because equipment failures can cause other equipment failures, quality problems, wasted material, and recalls. Seen in this light, replacing working but end-of-life equipment is a lot more economical than letting them fail and having an emergency.
I wonder if that same mentality applies to households, which rely on a wide range of equipment to run. Consider yourself to have bought a couple of weeks to get your supplies and tools lined up for a WH replacement project.
