International Harvester’s Gas Start Diesels

Making diesels user friendly was an uphill struggle in the late 1920s and early ‘30s. The advantages of the compression-ignition engine were generally known but the PITA involved in using one back then often outweighed those benefits. Part of the diesel reluctance was infrastructure related, such supply issues at the user level. Support for diesel powered equipment was few and far between in those days. Day-to-day, especially for hand-started engines, cold weather operation remained a big stumbling block.

A PD-40 engine mounted in Chuck Lehman’s 1935 WD-40 tractor. Don’t go looking for the changeover lever. It’s normally removed and stowed until needed. The yellow arrows indicate some key elements: 1- The timer control on the injection pump. 2- The stowed starting lever. 3- The starting mechanism shaft to which the starting lever is attached.

The International Harvester Answer

International Harvester (IH) started experimenting with diesels in 1916 with a single cylinder prechamber design concocted in their own shop. Not much came of it but in 1927 the Gas Power Engineering Department bought a 4-cylinder German Dorner diesel to evaluate. The Dorner was impressive enough that in1928, IH engineers made three other experimental test engines and eventually came up with their own design. In 1930, they bought and tested other makes of German-made diesels against their design and found it worthy of production. part of that process was also to design and later build their own fuel injection systems.

The PD-40 in it’s original mid-1930s form. The PD-40 had a 4.75 x 6.50 inch bore and stroke, delivering 460.7 cubic inches. On the diesel side, it was a prechamber style IDI engine with a 15:1 compression ratio. It was wet sleeved with five main bearings. The initial output was a maximum of 62.5 flywheel horsepower but the continuous rating was 50 horsepower at 1250 rpm.  PTO power on various tractors was around 48 horses. The PD-40 lasted about three years in its original form but was upgraded with an improved combustion chamber in 1936 at the same time a six-cylinder version of the engine debuted. Along the way, the main bearings improved from babbitt to copper backed lead.

Part of the base problem was to solve the starting issue. In large stationary applications, air start was the option. As engines got smaller, some of the smallest (mainly single cylinders) could be hand started but battery technology had not progressed far enough in the ‘30s to make electric start viable in most cases. Caterpillar, and others, used pony engines, a separate gasoline engine that ran long enough to heat the diesel a little and then spin it over. That worked very well but added a lot of bulk, expense and extra steps.

The original gas start system was fairly complex but worked well. Here is a key to the parts on a UD-40. A- Control lever for shifting from gas to diesel operation. 1- Injector. 2- Exhaust valve. 3- Intake valve. 4- Starting valve, connects the gas and diesel combustion chambers.

IH developed a system that combined a gas engine with a diesel. We’ll tell you exactly how in a sec, but the engine started on gasoline and then switched over to diesel after it was warmed up. In the long run, the gas start system was probably more user friendly, especially for equipment that had a lot of stop/start cycles, and it made the overall engine package more compact, and IH had the long term goal of adding diesel power to their smaller tractors. The pony-started engines were probably better diesels because they didn’t have to make any tradeoffs on the diesel part of the design. That worked for Cat because the majority of their equipment was bigger. It’s not so much a “which is better” question but more two roads to the same place, each route chosen to better serve the needs of each company.

This cutaway view show everything but the injection pump and give a better idea how it worked. 1- This spring-loaded lever opens the starting valve (3) to connect the gas and diesel combustion chambers and moves the gasoline system poppet valve from its lower seat (7) to it’s upper seat (9), opens the gasoline valve (6) and allows fuel into the carb (10) and airflow thru the intake passage (8) and into the combustion chamber. The release rod (2) operates a valve on top of the injection pump that shuts off diesel fuel flow. If the timer is engaged, when 700 revolutions are reached, it releases the mechanism and the engine reverts to diesel operation.

The Production Engines: PD-40 
and PD-80

The early IH diesels were initially based on IH heavy-duty gas engine architecture. This aided both the development and especially the manufacturing, where at least some of the gas and diesel components could be manufactured on the same tooling. The first production IH diesel was dubbed the PD-40 and debuted in April of 1933. If it was in a crawler, it would be called a TD-40. In a wheeled tractor, the WD-40. A power unit would be a UD-40. If in an Industrial tractor, it was the ID-40. The engine designation was also the tractor designation in this era.

The UD-18 six in 1942 after the evolution to the new combustion chamber and a new gasoline start system. It’s mounting an American Bosch APE injection pump. The gas start system was revised and the poppet valve connecting the gas intake to the diesel intake was replaced by a flapper type valve. The gas engine compression ratio increased to 6.5:1 for better starting and the engine was started by an optional electric starter, though the hand crank system was still available in certain applications. Most of the UD-14 and UD-18 engines used Bosch injection pumps, while still using IH built injectors. IH would later build a revised pump of their own for these engines.

The big innovation was the gas start feature. Look at the nearby illustrations for more detail, but the essential feature of a gas start diesel was an auxiliary combustion chamber connected to the main diesel chamber by what was called a starting valve. When that was opened, the added volume of the auxiliary chamber dropped the compression ratio from 15:1 to about 5:1 and it contained a spark plug fired by a magneto. The gasoline intake tract was connected to the diesel system via an air valve. A small 3/4-inch bore carburetor supplied enough air and fuel to run the engine at about 400 rpm. A lever connected to various linkages opened or closes valves to the various chambers and disabled either the magneto or the injection pump. It worked better than you might think, especially when viewed in the context of the era when “diesel” and “cold starting” were exclusive terms.

Compare the details on the revised gas start system to the earlier one. This system is more or less what was used for the remainder of the IH gas-start diesel era. 1- control lever. 2- Gas start linkage which operated the starting valve (3) that connects the diesel combustion chamber and the gas combustion chamber (4). This linkage also operates the flapper valve (6) in the intake manifold that directs air through the carb (8) or the large intake runner (7) and a fuel shut off valve at the carburetor. This later system offered electric starting and the controls were all accessible from the driver’s seat in the engine was in a mobile application. With a strong battery, it was possible to start a fully warmed up engine with the electric starter while in diesel mode.

The first generation IH diesels would be manually switched over to gas engine for starting and hand cranked. No, these first engines did not have electric start. The injection pump had a timer that counted the number of engine revolutions (about 700) and it could be used optionally to automatically trigger the system to switch back to diesel operation after approximately two minutes running on gasoline. In most cases, two minutes of running was enough to warm the engine enough to run on diesel but the operator could run the engine on gas as long as needed.

The MD was powered by the UD264 and this is a 1941, the very first year for that application. You can see both sides of the engine, the diesel side showing the new IH injection pump and the gas side showing the tiny carb and distributor. Tucked away, you can also see the spark plugs. The basic design lasted to 1960. If you want to see an MD start on gas and switch to diesel, check out the video at


A six-cylinder diesel began development in 1933 and was released for production as the PD-80 in February of 1936, debuting in July. It was essentially the PD-40 with two cylinders added. It was a stout seven main engine with wet sleeves. It did have some design evolution over the PD-40, notably an improved combustion chamber. Making 691 cubic inches from the 4.75 x 6.50 bore and stroke, the engine cranked out a maximum of 100 horsepower and 80 continuous horsepower at 1400 rpm. It didn’t find a home in tractors but was offered in a power unit starting in 1937.

The PD-35 engine was a short-lived variant of the PD-40 that only appeared in the ‘37-39 TD-35 crawler. With a bore 1/4-inch smaller than the PD-40, it made four horsepower less, as measured in Nebraska tractor tests. As far as we can see, the PD-35 engine was not offered in anything but the TD-35 crawler, which was marketed as a “budget” TD-40.


The TD-40 and TD-80 evolved in 1939 with a large number of improvements. The evolution had begun with the 1936 updates mentioned. The original engines were basically sound and reliable, so the main thrust was improving combustion efficiency and increasing output. Major changes occurred to the cylinder head, combustion chambers, fuel injection and the way the gas start system operated. As such, the UD-40 engine became the UD-14 and the UD-80 became the UD-18. The UD-18 found a home in the TD-18 tractor at the end of 1938, making it IH’s most powerful crawler for a time. Initially, the maximum output of the four-cylinder engine increased to 81 horsepower maximum, 68.5 intermittent and 54 horsepower continuous. The six jumped to 119 max, 100 intermittent and 80 continuous. The power increases were not gigantic but efficiency and economy was much improved.

Here is a 1942 UD-18 as mounted in a military TD-18 crawler. The American Bosch APE pump is prominent. You can see more about this crawler on the Diesel World website

In 1946, another upgrade was released for production, the UD-14A and UD-18A, incorporating more improvements in breathing and combustion chamber design. The result was a maximum of 90.5 horses for the UD-14A and 150 horses for the UD-18A. This series of engines would remain in production into the early ‘60s. The UD-14A is still seen in the 1960 engine catalog but not the UD-18A, which ended in 1958, replaced by more modern six-cylinder designs.

New Kids: UD-6, UD-9 and UD-16

In 1939, two smaller gas-start, four-cylinder diesel engines were released for production, the 248 cubic inch UD-6 (3.88 x 5.25-inch bore and stroke) and the 334 cubic inch UD-9 (4.40 x 5.50-inch bore and stroke). The UD-16 was a six-cylinder variant of the UD-9. Both the fours were available in tractors or as power units. Following previous convention, the UD-6 appeared in the 1940 TD-6 crawler and the  WD-6 wheeled standard tractor and in the 1941 Farmall MD, International’s first rowcrop diesel.

The two faces of the 335 cubic inch UD-9 diesel as installed in a TD-9 crawler. The UD-9 made a max of 63 horses but the continuous rating was 53 at 1500 and 230 lbs-ft of torque at 1200 rpm. It saw use in the midsized TD-9 crawler and the WD-9 standard tractor that more or less replaced the WD-40 in the standard tread upper weight class.  A little remembered UD-16 variant used the same design but with two more cylinders. It made 501 cubic inches and a max of 118 horsepower.  It evolved with the same bore increases as the UD-9 and grew to 525 and later 554 cubic inches.

The UD-6 shared a lot of features with the gas and distillate engines that had debuted with the new model M wheeled tractors and the W6 that had debuted for 1939. These new fours were dry sleeved with three main bearings. The new UD-6 delivered a maximum of 45 horsepower at 1500 rpm, though the continuous rating was only 39 horsepower. This series engine used a new IH injection pump that debuted at the same time.

The UD-6, UD-9 and UD-16engines were updated in 1953 with bore increases, the UD-6 to 4 inches and the UD-9/UD-16 to 4.5. This bumped the displacement to 264 and 350/525 cubic inches respectively. With some accompanying tuning and a 100 rpm bump in the redline, the UD-6 delivered a max of 54 horsepower and the UD-9 78.5. These engines saw use in the “Super” versions of the MD, WD-6 and WD-9, as well as with power units. They also appeared in the updated tractor lines like the 400 Farmall from 1954-56 and the W400 standard.

The final evolution of the UD-6 engine line came in 1956, with another bore increase to 4.125 inches. This bumped the displacement to 281 cubic inches. As far as we can determine the TD-9 engine continued on as it was until it was discontinued in 1958.

The Biggest Boy: UD-24

The biggest gas start IH diesel was the UD-24, used in a power unit and the TD-24 crawler starting in 1947. It was a beast, making 1,091 cubic inch from a 5.75 x 7.00 inch bore and stroke. In the big IH TD-24 crawler (built 1949-1959), it made 146 horsepower on the drawbar. Around 1955, IH adopted new terminology and renamed this engine the UD-1091 and it was offered as a stationary unit at least through 1960.

The Final Word

IH got a lot of mileage out of their gas start designs and even late in their 30 year run, when the diesel industry was finally getting a handle on cold starting issues, those so-called “all-weather diesels” still had a strong place in the market. Their design left a few horsepower on the table compared to what came later but even as improved IDI and DI engines came on the market, a lower-power diesel that starts on that cold and snowy day is better than a higher power one that doesn’t.

The mighty UD-24! With 1091 cubic inches, engine was rated at a maximum 191 horsepower at 1400, with the intermittent rating at 180 ponies. The continuous rating was 144 horsepower at 1375 rpm. Like all the other IH diesels of the day, it was dry sleeved. It’s shown here in the UD-24 power unit form but was also used in the ‘47-59 TD-24 crawler.


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