Series 4J & 6J, 1938 - 1942

The R2 is the spark ignition version of the popular D2 tractor, but please note that Caterpillar also made a small number of tractors designated R2 from 1934 to 1937 with serial numbers 5E3501 – 5E3583. These were basically the same as the Cat Twenty Two (see separate Cat Twenty Two page).


Production began in 1938 and ran until 1942. Only 1185 4J and 1150 6J tractors were produced as the diesel engined D2 was much more popular and economical.

The 4J version has a width (gauge) of 44 inches and the 6J a gauge of 50 inches, corresponding to the 3J and 5J D2 tractors respectively. Being a spark ignition tractor, it had a starting handle at the front like all other small gas tractors and no pony motor.

Most if not all tractors were equipped to run on “Tractor Fuel” rather than straight gasoline, the tractor fuel being less expensive than gasoline. When running on tractor fuel, the tractor was started on gasoline and warmed up before being switched over to tractor fuel. “Tractor Fuel” is known in the UK as “TVO” (Tractor Vapirising Oil) and in Australia as “Power Kerosene”. TVO is no longer made but a substitute can be made by mixing 4 gallons

of 28sec heating oil, 1 gallon of petrol (gasoline) and a quart of diesel.


The space occupied by the pony motor, just in front of the operator’s seat, on the D2 accommodated the fuel tank. The fuel tank itself is a complex affair with a separate internal gasoline tank. This is basically the same rectangular gasoline tank as used on the D4 7J tractors but affixed inside the main fuel tank with it’s own separate filler cap. To further complicate matters, the gauges and electrical switches are mounted in a recess in the rear wall of the fuel tank with a tube going front to back through the tank to carry the necessary cables and temperature gauge capillary tube.


The R2 engine is designated 3400G (The D2 J series tractors used the D3400 engine) and many of the engine components are the same as the D3400.

You can find the series designation on the serial number plates which are aluminum plates located in two places, at the top rear left hand side of the engine block and at the top left-hand side of the rear of the transmission case.

Many R2 tractors were shipped to the UK under the wartime “lease lend” scheme. It is not uncommon to find in the UK what appears to be a D2 tractor which, on closer inspection has a 4J or 6J serial number on the rear transmission case and a 1S serial number on the engine. These tractors have been re-engined with a D3400 engine from a wartime generator set.

Comparison of the R2 and D2 tractors

R2 J SeriesD2 J Series
1938-1942 1938-1947
3400G Spark Ignition Engine D3400 Diesel Engine
25 D.H.P. 28.8 1525 rpm
Weight 6,350 lbs. Weight 7,730 lbs.

Below is a list of machines made by year, model and serial number.

1938 1 1 1 4
1939 2 746 5 629
1940 747 1009 630 871
1941 1010 1172 872 1130
1942 1173 1185 1131 1150

Below is a picture of a restored R2 with her grandchildren !


The members here at ACMOC have a wealth of knowledge on Caterpillar machines and machinery.

Join today and you'll find digitized original manuals, historical photos, helpful members, and a wealth of information whether you have an R2 or are thinking you need one like the rest of us here at ACMOC, and trust us, you do need one.

The ACMOC Members Library contains Operation and Maintenance Instructions books, Parts Catalogs and Serviceman’s Reference Manuals, as well as links to YouTube videos showing detailed restoration of these popular tractors.

Created by John Gaunt

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