Classic Machines Tractors

Classic Machines: The Allis-Chalmers HD-21 tractor

Developed directly from their model HD-20 (introduced 1951), the HD-21 featured among other things, one of the diesel engines from the recently acquired Buda Engine Company. By Richard Campbell

Introduced in 1955, the HD21, an 84 inch gauge, D8 sized tractor, was the flagship of the company’s track type tractor range for almost 14 years

Prior to 1953 Allis-Chalmers did not manufacture their own diesel engines, relying mainly on GM for its motive power.

This all changed when Allis-Chalmers purchased Buda and began installing them in their tractors (suitably re-badged of course).

The HD-21 was a very well designed and balanced tractor with ample power and was highly regarded, especially in forestry applications.

All of the first production examples featured a supercharged Allis-Chalmers HDS844 6-cylinder diesel engine which put out 204 flywheel horsepower and had a 2-speed torque converter drive transmission.

It was not too long before Allis-Chalmers also introduced a power shift transmission as an option. This later became standard.

Allis-Chalmers uprated the engine in 1956 to a turbocharged model HDT844 which produced 225 flywheel horsepower and this was the last of the HD21 engines to use the Lanova fuel injection system.

The first of the true Allis-Chalmers engines was introduced in 1959, the 225 horsepower model 21000 which remained the standard engine until it was replaced by the more powerful series 2 type in 1967.

Other various refinements of the design took place up until 1969 when the HD-21B was introduced.

This featured the model 21000-II engine rated at 273 horsepower and was completely restyled in the panelwork department, having straight angular lines compared to previous versions of the tractor.

Fiat entered the scene in 1973 and not long after that the model 21C was introduced.

The 21C proved to be the last version of the HD-21, the type disappearing from the sales catalogue in 1981 to be replaced by the short-lived FD20.

It is interesting to conjecture what would have become of Allis-Chalmers if Fiat had not had a controlling interest in them as Allis-Chalmers had always been very innovative and forward thinking company. It is a pity that they are now relegated to history.

The HD-21 Described

We will examine the first of the line in this feature, a 1955 HD-21A.

As mentioned earlier, the heart of the machine was an A-C branded Buda HDS-844 six cylinder supercharged diesel rated at 204 flywheel horsepower.

This was connected to a 2-speed manual transmission via a diesel charge torque converter which gave the HD-21 a top speed of approximately 7½ mph in top gear.

Final drives were of conventional spur gear type, double reduction, with hydraulically boosted multiple disc steering clutches and contracting band brakes.

The track frames held six bottom rollers and two carrier rollers per side and followed conventional tractor design of the time. A pinned equalizer bar tied the two frames together in the centre of the tractor and allowed a vertical oscillation of 20°

Standard track shoes were 24” wide with many optional sizes available.

Compared to the previous model HD-20, the operators area had been improved with a much better seat and attention to operator visibility.

Allis-Chalmers were the first large tractor manufacturer to introduce a tapered bonnet on their track type tractors for better visibility to the blade.

Deeply upholstered seats were always a feature of Allis-Chalmers tractors and the HD21 was no exception. The machine also included a smaller ‘buddy’ seat for a trainee or relief operator.

A small instrument cluster was placed to the right of the steering lever console and this held oil pressure, water temperature, converter temperature and ammeter gauges.

Master engine clutch was in the usual position to the operators left and the only items to penetrate the floor decking were the two brake pedals and gearshift lever.

Unusually for a tractor of the 1950s, the HD21 was fitted with a muffler as standard equipment!

Comfort options included an insulated cab with a heater and wipers and various lighting arrangements.

Work Attachments

Initially, Allis-Chalmers relied on a number of outside suppliers for bulldozer blades, winches, cable controls and rippers. Companies producing these included Baker, GarWood, Tractomotive, Ateco, Pullman-Standard, Hyster and Carco.

Generally speaking, Baker supplied the hydraulic blades and GarWood the cable activated variety.

Both Hyster and Carco supplied logging winches for use on HD21’s but by far the most preferred type on an HD21 appears to have been Carco.

Ateco and Tractomotive supplied rippers of various types while Pullman-Standard and GarWood supplied the cable controls.

By the late 1950s however, Allis-Chalmers were manufacturing almost all of their own attachments apart from winches.

The New Zealand Connection

Regrettably records no longer exist for just how many Allis-Chalmers HD-21’s were imported into New Zealand by dealer Cable-Price.

Your author estimates over 70 of all types have made it to New Zealand, the majority of them during the 1950s and 1960s.

Users included Egmont Land Development, Ray Ordish, Downer & Co,(who had lots), NZ Forest Products and Isaac Construction to name but a few.

HD21’s have been used throughout New Zealand and the odd example can still be found working.

For the Diecast Model Collector

Models of the HD-21 are available in both 1:50th and 1:25th scale and fortunately for the collector, both are manufactured by First Gear.

The 1:50th scale model is available in 3 versions and is of very high quality with metal tracks and all the bits and bobs a real HD-21 would have. It represents an early 1960s variant with powershift transmission.

In 1:25th scale First Gear’s model is of Allis-Chalmers’ first production version of the tractor, the HD21A with the supercharged HDS844 engine. It features a working cable operated angle blade with rear mounted GarWood 241 cable control.

Well worth adding to any large tractor collection, the model is of exceptional quality and is absolutely exquisite.

Brief Specifications – 1955 Allis-Chalmers HD21A

Engine: Allis-Chalmers model HDS844 6-cylinder, inline, supercharged diesel

rated at 204 flywheel horsepower at 1800 rpm

Transmission: Allis-Chalmers manufactured torque converter drive transmission with

two forward and one reverse range

Top Speed:     7½ mph

Final Drive:     Double reduction spur gear

Steering:          Multiple disc cltches & contracting band brakes

Track Gauge:   84”

Track Chain:   Sealed, 37 section

Rollers:            6 bottom, 2 top per side

Std.Shoes:       24”

Ground Press: 8.6 psi

Length:            16’ 3” (bare)

Width:             9’ 1¼” (bare)

Height:                        9’ 4” (to top of stack)

Op.Weight:     22 tons (bare), 25½ tons with S-blade & PCU

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