I ran numerous stationary engines for a water company years ago. We had a number 455 CI Oldsmobile engines sitting out in an open field running irrigation water pumps. These engines ran 24/7/365 under heavy load and were never shut off until they broke down.
They ran on N/G, so they had a never ending supply of fuel. There was a 25 gallon reserve oil tank sitting next to each engine, which was connected to the oil pan via a flow gauge which maintained a constant oil level in the crankcase.
These engines were specially built to perform for as long as possible. They had all of the high performance tricks that any racing engine would have. They engines were built for longevity rather than speed, so no Hi P cam or pistons, etc. They weren’t build to go fast, they were built to go long.
A lot of special attention when building a dry fuel engine not necessary for a wet fuel engine, much more labor intensive as so many little details need attention to in order to live with dry fuel. The weakest link was the valve train, the valves would slowly grind the seats away due to no lubrication in dry fuel, so special.
Hardened valve seats were required and would still eventually grind there way right through the seats and into the head. The cranks were tuffrided and the assembly balanced, no expense was spared to build the perfect, long lasting engine.
In answer to the question, how many miles the engine the engine would last could not be measured because it was a stationary engine, it lasted zero miles, however they would last usually 3 or 4 months running 24/7 at a constant RPM, under heavy load. Imagine driving straight up a steep mountain road pulling a heavy trailer for four months non stop and you get the idea.
A diesel would most likely have lasted much longer but a diesel would have required a large fuel tank that would have to be refueled frequently, with N/G the fuel supply was unlimited. Also a diesel would have required a much more difficult installation process and refueling process due to the remote engine location, out in the toolies..!
Back in 1959, two pilots flew a plane continuously for 64 days, 22 hours, 19 minutes. They were refueled by other planes so effectively had unlimited fuel. To keep the engine running, they had to regularly add oil as others have mentioned. Big diesel generators can run for months and months without any maintenance and are designed to keep the lubricating oil there and (in many cases) to swap it out over time while the engine is running.
For a gasonline engine the first failure (assuming proper lubrication by some method) would likely be the spark plug electrodes getting covered with carbon. On a modern engine with good tolerances, the bearings and rings effectively will run for years if they get the right lubrication.
If the valves aren’t adjusted correctly, one of them will start leaking by (losing power) but I’m not sure that would be a failure. Sparkplugs getting to the point where one or more isn’t sparking would be the most likely failure point. This is one of the reasons Diesels are so reliable and can run for so long…no spark plugs!
It has been pointed out that my memory was faulty. The plane in the air refueled from trucks/cars with fuel on the ground and they pulled a hose up and got the fuel, not air-to-air refueling, but it doesn’t take away from the base answer (and consensus in comments below). Oil tends to be the key thing that, if not replenished and changed regularly) leads to engine failure.
In the 1960s the US Air Force had remote weather stations in the arctic. These were powered with 10 hp Briggs & Stratton cast iron industrial engines that were supplied with enough fuel and lubricating oil to run for a year. Each set had a backup motor/generator set.
I worked with an Air Force vet who worked on these weather stations. He reported they never had a failure that required the back-up motor.
When they serviced them, they would exchange the unused set with the 8760 hour engine, which were then checked and rebuilt and replaced, as the back-up. With a constant flow of clean oil, the engines exhibited virtually no wear except for the exhaust valve, seat and guide. This is the highest heat area of any engine. The rebuilds consisted of a new exhaust valve, seat and a new set of points, condensor and spark plug.
With all proper lubricants and fuel, a modern gas engine will run constantly and will fail at the critical high heat area, the exhaust valve.
I remember a story from a Diesel engine class in college.
A hi-way tunnel was built through a remote mountain that required electrical power to operate lights and fans. Two Diesel generating systems were procured to provide this power. One was primary and one back up. The engines were designed to be serviced while they were running. This included changing lubricating oil and oil filters, fuel filters and air filters.
The primary engine was started and brought online and carried the load. After a little over 11 years (100,000 hours of continuous operation) it was felt that the primary engine would require overhaul. They contacted the builder and arranged for the overhaul.
They came and got the engine and took it back to their factory and began the overhaul, after a little while they contacted the tunnel authority and made them this offer. They would trade them a new engine as a direct trade, no other costs for their old engine.
The tunnel authority was happy with the deal but asked why. The manufacturer said they disassembled the old engine and inspected it and could find NO ware all components were at factory specifications. They wanted to use it in their advertising.
The engine was run at a constant rpm, temperature and load. The air, fuel and oil were kept clean and kept at the proper pressure. When you do this there is almost no metel to metel contact, thus no ware.
I think the most risk of failure would be a part failing somewhere in the engine due to fatigue.
My story was about a diesel not gas, for a gas engine I think the weak point over time is ignition specifically the spark plugs.
