1955 International Harvester UD-18A

International Harvester is well known for tractors, crawlers, combines and construction equipment, almost all powered by engines built in-house. For the better part of a century, IH, and its corporate descendant Navistar, was a renowned engine builder. Not only did they power their own wheeled equipment, but they also sold engines all over the world for every conceivable purpose, many tailor-made for specific applications. Because heavy-duty engines suitable for tractors and construction equipment are also generally suited to stationary applications, much of the engine line back in the day was seen in several venues. That included the 691 cubic inch UD-18 six-cylinder gas-start diesel.

1955 International Harvester UD-18A

1955 International Tractor
By the serial number, this is a 1955 unit. It was in continuous use for nearly 20 years, acquiring over 9,100 operating hours. One of the most amazing parts is that it still has the original side covers, much of that being the same as what was used on the TD-18 crawlers of the era. The starting sequence for the gas-start diesels requires the covers to be removed and, very often, operators find some corner to stash the covers and usually they get lost. It’s mounted on the original steel bed, which was once part of the asphalt plant. Note the dual stacks. The engine has two cylinder heads, each covering three cylinders and each with a separate exhaust manifold and stack. Dewey added the axle, wheels, hitch, and jack to make the unit more portable.

Crawler Roots

The 691 cubic inches, six-cylinder IH diesel debuted in July of 1936 as the PD-80. It was a 6-cylinder first cousin to the PD-40, a four-cylinder that had emerged in 1933 to power America’s first production diesel-powered wheeled tractor, the WD-40. The 100-horsepower PD-80 did not immediately find a home in tractors or crawlers but was sold as the UD-80 for power units and other stationary applications. It began an evolution almost immediately and an updated variant debuted at the end of 1938 in the TD-18 crawler, which became the International’s largest. The main thrust of the evolution was to improve combustion efficiency, so the cylinder heads and injection system had been much improved. The engine designation also changed to UD-18 in stationary units and TD-18 in crawlers.

Maximum continuous was 100 horses at 1600.
The diesel side of the engine shows the IH-built two-plunger injection pump that debuted in 1946 with the UD-18A series engines. In the year this UDR-18A was built, the bare engine was rated for a maximum of 131.5 horsepower at 1650 rpm. In power unit form, the maximum intermittent rating was 125 horsepower at 1600 rpm. Maximum continuous was 100 horses at 1600. The Rated torque was 462 lbs-ft at 850 rpm.
A cutaway of the twin plunger injection pump.
A cutaway of the twin plunger injection pump. Each of the high-pressure plungers feeds three cylinders. A similar single plunger pump was used on four-cylinder engines. This pump actually predates the engine. By serial number, the pump in Dewey’s engine was built in 1947. Either it sat around before being installed in 1955, or the pump was replaced by a reman at some point in the engine’s life.

Research shows the first UD-18 power unit was built in January of 1940 and it debuted as the big boy in the lineup. World War II soon overtook IH, as it did with most American industry, but by the end of the war, the IH engineers had worked out enough improvements to bring forth UD-18A in 1946. A good deal of the improvement came from a new twin-plunger injection pump but there were also further improvements on the combustion side and many small durability and serviceability updates.

A Long Production Run

If you count the original ancestor, the PD-80, and the final variant, the UD-691as part of the family, this engine was in production for 31 years, from 1936 to 1964. The UD-18A had the largest number produced, with 8,944 units listed from ‘46-59, the UD-691 listing shows 500 were built from ‘59-64. The UD-18A’s big cousin, the monstrous 1091ci UD-24A (aka UD-1091), lasted a bit longer and was built in 1965. Other siblings included the UD-14A, which shared the same bore and stroke as the UD-18A, and the UD-525, which was a six-cylinder that shared the same 4.44 x 5.50-inch stroke as the original 460 ci WD-40 diesel. By the middle 1960s, the gas start feature was becoming a bit clunky. Better diesels, better electrics, and glow plugs offered more efficiency and less complication.

The gas side of the engine shows the carburetor, divided intake manifold, spark plugs, and distributor.
The gas side of the engine shows the carburetor, divided intake manifold, spark plugs, and distributor. Engines used on crawlers and other mobile equipment usually had a magneto instead of a distributor and coil so the distributor mounting hole was capped.
Compression Ration Diagram
Here is how the gas start system worked on the UD-18A. On the left is the engine in gas mode. The red areas show the gas intake system and combustion chamber. The starting valve (third valve) is open, connecting the gas combustion chamber with the diesel side and that drops the compression ratio from 15.5:1 to 6.5:1. The flapper valve is closed in the diesel runner of the intake manifold. The spark ignition is energized and the flow valve allowing fuel to the carburetor is open. On the right, diesel mode is shown and with the starting valve closed, the compression is back up to 15.5:1, the intake flapper is open and the engine is drawing air through the main runner. The ignition is de-energized and the carb is not getting fuel.

A Trio of Rock Crushing Diesels

In the mid-1950s, The National Lime & Stone Company acquired at least two and probably three UD-18A power units. At any given moment, two of the engines were powering an asphalt plant. If one engine went down, they would quickly swap it out with the third engine to keep the plant running. It’s known from the current owner, J. Dewey Hetzel, that two of the engines have consecutive serial numbers. He owns both those engines but the third got away. While he thinks they were all bought at the same time and are likely consecutive, he can’t prove it. We consulted with National Lime & Stone and they couldn’t offer any more information. The Carey quarry has a long history, going back to 1903 and the beginning of National Lime & Stone. The engines were in use into the early 1970s. When the asphalt plant in Carey, Ohio, quarry was taken down in the early ‘70s, Dewey was on hand to take two of the power units for his collection of IH equipment.

The hour meter shows 9137 hours.
The hour meter shows 9137 hours. That’s a lot of hours on an engine, equivalent to about 550,000 road miles on a vehicle, but considering about 20 years of service, that’s only about 450 hours per year.
The simple control panel has survived 69 years so far.
The performance chart of 1955 shows the maximum, intermittent, and continuous ratings, as well as the fuel consumption. At its maximum power of 131 hp, it used a pretty reasonable 8.4 gallons per hour.
Red Power Round Up in Springfield, Ohio
We wondered what an asphalt plant looked like and found this image that dated to 1953, same as the engine, but we don’t know if it was the same make of plant as used in Carey. We reached out to National Lime & Stone for info on the asphalt plant used at their Carey, Ohio, gravel pit but they couldn’t come up with details by press time. They haven’t mixed asphalt there in a long time. The Carey pit is the one that started its business back in 1903 and the company is proud to still be privately owned and flourishing nearly 123 years later.

The power unit shown here was seen at the Red Power Round Up in Springfield, Ohio, in the summer of 2022. When Dewey demonstrated the unit for us, it quickly drew a small crowd of people. It’s still hale and hearty. At over 3,000 pounds as you see it here, moving it around is no cakewalk but visitors were appreciative of the opportunity to see a working example of a bit of Red Power engine history.


The National Lime & Stone Company

Red Power Roundup


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