Caterpillardual axleScrapers

Caterpillar’s DW21 scraper revisited

Born of necessity, the Caterpillar DW21, the great granddaddy of all Caterpillar 2-axle scrapers, went on to become a very successful scraper for Cat and also laid the groundwork for the later two-axle 600 series machines. By RICHARD CAMPBELL.

We originally had a look at the Caterpillar DW21 back in May 2004. As with some of those articles that were written in the early 2000s, a lot more information has become available and I’m also able to share a wider photographic perspective of these machines nowadays.

The people at Caterpillar realised with hindsight that they had missed a great opportunity when turning down RG LeTourneau’s proposal to build a self-propelled scraper in the early 1940s.

As a result, LeTourneau practically had the whole self-propelled scraper market to itself following the war, its only competition being Heil, LaPlant-Choate and Wooldridge, none of which had the large manufacturing capacity of LeTourneau.

Caterpillar could undertake very little research and development work on new products during the war years, its primary task was to supply track type tractors and motor graders to the military in quantity, so its hands were somewhat tied.

However, during late 1944 when the tide of war had obviously turned in the Allies favour, resources were made available to begin design on what was to become the DW21.

It took until 1948 to produce a working prototype.

At the same time work was also underway on a three-axle machine to be known as the DW20.

Both of these machines were powered by Caterpillar’s newly developed D337 six-cylinder engine, a feature of which was a Rootes blower to pack more combustion air into the cylinders. The early D337 was unique as the only Caterpillar diesel engine to be ever manufactured with a blower.

There were basically six versions of the DW21 built by Caterpillar over the period of its development.

Commencing with the 8W series in 1951, this featured the No 21 cable scraper with a capacity of 15 cubic yards struck and 19.5 cubic yards heaped.

The aircleaner was mounted on the left side of the machine in front of the operator with an angled fuel tank mounted on the machine’s right fender. This version had a single exhaust pipe. Engine output was 225 flywheel horsepower and the machine weighed approximately 24 tons empty.

Problems with exhaust back pressure causing cylinder head failures and valve ‘floating’ led Caterpillar to modify the cylinder head and exhaust manifold to a twin stack design around serial number 8W1061.

A complete reworking of the machine’s fuel tank allowed the aircleaner to be shifted from in front of the operator and fitted in a notch set into the fuel tank on the right hand side. This also reduced the amount of air intake piping required.

Heaped scraper capacity was also increased to 20 cubic yards.

With these modifications the machine’s empty weight rose to 27.5 tons.

All of the 8W series DW21s had Caterpillar’s 2-cylinder gasoline pony motor starter for the D337 engine.

Ongoing problems with the D337 engine led to the next version of the machine, the DW21C, introduced in 1955.

The tractor unit was redesigned and a turbocharger was fitted to the D337 engine, replacing the Rootes blower.

Installation of the turbocharger increased engine output to 300 flywheel horsepower.

A wider range of tyres could also be fitted with the 29.5×29 wide base type supplanting the 24×29 of the original DW21.

A new scraper was also provided, the ‘lowbowl’ No 470, which had a capacity of 18 cubic yards struck and 25 cubic yards heaped.

Empty weight was now 29 tons.

For the first time, the D337 engine was available with a direct electric starter motor as well as the traditional Caterpillar pony engine.

Caterpillar saw fit to make a distinction between the two starting methods calling the direct electric start model a 58C series and the gasoline starting engine model the 69C series.

The next version of the DW21, the DW21D, was introduced in 1958.

Horsepower was increased to 320 flywheel horsepower and the front of the tractor unit now had a perforated steel radiator guard replacing the former models’ wire mesh arrangement.

Capacity of the No 470 scraper remained the same.

Caterpillar identified the DW21D as the 85E series with electric starter motor and the 86E series with gasoline pony motor starting.

Around 1959, further changes were made to the DW21 with increases in horsepower and scraper capacity.

These alterations resulted in the machine designation being changed to DW21G, however, despite these changes, Caterpillar curiously did not alter the serial number sequence, the machines remaining as the 85E and 86E series.

The D337 engine received a new turbocharger, boosting the flywheel horsepower to 345 and the scraper was upgraded to the No 470B model with a struck rating of 19.5 cubic yards and a heaped rating of 27 cubic yards.

Increasing the horsepower necessitated adding twin aircleaners to cope with the engine’s demand. This is a good spotting feature when trying to identify between DW21C, DW21D and DW21Gs as the Gs are the only ones with two aircleaners.
Heaviest of all the DW21s at 30 tons empty, the DW21G was the final production model of the machine.

It was replaced by the Caterpillar 631A in late 1960.


All of the DW21 models featured a manual (constant mesh) transmission.

On the early 8W series machines, this was a 5-speed forward and reverse type designed and built by Cat with a 16″ double plate, air-boosted clutch.

Later variants only featured 5-forward and 1-reverse ranges.

‘Gun’ operators could effect gear changes without using the clutch by momentarily activating the cable control unit which temporarily lugged the engine down enough to change ranges. This process was known as “winch changing” and was fairly widely employed.

Caterpillar also experimented with a semi-automatic transmission known as Synchro-Touch, which allowed the operator to dial up the gear required and electric solenoids would do the rest.

However, the system was prone to condensation damage resulting in all manner of gearbox failures and was quickly and quietly discontinued, while those units equipped with the device were converted back to the standard manual transmission.

The New Zealand Connection

New Zealand had a very healthy population of Caterpillar DW21s with over 30 being imported. These represented almost all of the sub series that Caterpillar built of the type, including the early single and double stack 8W series machines.

By far the largest operator of the type was W Stevenson & Son which used a giant fleet of them to remove overburden from the Kopuku opencast mine in the Waikato.
Such was the high standard of maintenance carried out by “Stevies” that most of the fleet lasted well into the early 1980s before being retired to second-line duties.

Other major users included Green & McCahill, McBreen-Jenkins and the late Jack Trenouth, who had an immaculate set of machines.

For the Model Collector

The Caterpillar DW21 is a machine crying out for a decent model to be made of it.

As of the date of publication there are two models available, one in 1:40 and the other to 1:87 scale.

The 1:40 scale model was manufactured by Revell in the early 1950s and while not being entirely inaccurate, is extremely basic. It was intended as a giveaway from Cat dealers to customers and their children.

Revell’s model depicts an 8W series machine and examples can be found on Ebay for under US$100. It is more of a curio for collectors rather than a true scale model.

German model maker Roco turned out a model of the DW21 to 1:87 (HO) scale in the early 1960s. It was also released by Austrian company Umex.

Roco’s model depicts a DW21C but has quite a few inaccuracies, especially in the scraper itself, which require a bit of work to turn the model into a decent replica.

So, until one of the large dedicated manufacturers such as Classic Construction Models takes notice and fills the void, those I’m afraid, are your options.

Brief Specifications – Caterpillar DW21C (the most numerous model)

Engine: Caterpillar D337T 6-cylinder turbocharged diesel rated at 300 horsepower @ 1800 rpm

Transmission: Caterpillar 5-speed constant mesh manual transmission

Clutch: Caterpillar 16” double plate with air booster

Top Speed: 20.5 mph

Brakes: Shoe type brakes on all wheels, fully air operated and synchronised to brake scraper first to prevent jackknifing

Steering: Hydraulic using 4x cylinders

Turn Circle: 36′

Tyres: 29.5 x 29, 22-ply E3

Scraper: Caterpillar No 470, fully cable operated via a 2-drom Cat No 27 PCU

Capacity: 18 cubic yards struck, 25 cubic yards heaped

Length: 41′ 7″

Width: 11′ 9″

Height: 11′

Op Weight: 29 tons (empty), 56.5 tons (loaded)

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