The Prototype -
Allis-Chalmers produced the HD-21 from 1954 to 1969; and to 1975 with a change in body style and specs as HD-21B. The HD-21 model included seven different variations of design: different body styling, power ratings, weight, air-filter canister placement. The HD-21 was the first Allis-Chalmer's model to use it's own manufactured engine based on the Buda diesel and not on Detroit diesel truck engine. Equipped with a 6 cylinder supercharged engine producing between 204 fwhp (1954 models) to 273 fwhp for 1963 model. They weighed in at 44,000 lbs (1954) to 57,100 lbs (1969 model) without any blade attachment. This means a 70-ton railroad flat car can carry a maximum of 2-3 tractors, depending on loading space and any additional attachments.

Historical Context -
The HD-21 directly competed with the Cat D-8. The HD-21 was a little cheaper in cost to acquire than an equivalent Cat machine. It became Allis-Chalmers all time best seller. The HD-21 was equipped with superior track bearings that kept out the dirt which the Caterpillar finally adopted in 1958. The HD-21 had a difficult to use manual transmission prior to their all power shift transmission which came later. Until the 1960, the dozer model blade was operated by cable and winch. From the 1960's on, Allis-Chalmers incorporated a hydraulically operated blade. The production of HD-21 pre-dated the 1975 OSHA law that required mandatory roll over protection (canopy. However, owner-operators and contractors sometimes added an after market canopy. In 1975, Fiat SpA took over Allis-Chalmers line of tractors that was rebranded under the Fiat-Allis name.

Wheels of Time Custom Editions Model -

HD-21 Crawler Kit is especially like how they are handled on railroad flatcars. The Crawler Kit consists of three piece (body, right side and left side treads) high quality urethane resin castings in neutral gray. Exhaust pipe is made of 3D printed acrylic. Assembly is required!

HD-21 Dozer Kit consists of body, right side and left side treads. Made of high quality urethane resin casting in neutral gray. Highly detailed two-piece blade (for easy installation on tractor), exhaust pipe and hydraulic cylinders in 3D printed acrylic. Assembly is required!

Requires washing model in dish soap, rinsing in water and letting it dry to remove any model oil release. The 3D printed acrylic should be washed in Simple Green and gently scrubbing with a toothbrush to remove any wax. Then rinsed thoroughly in water.

The HD-21 early productions were painted in orange before Allis Chalmers switched to yellow.
Paint matches: Model Master laquer based paint - Orange (990 - Hugger Orange - 1969 GM cars); Yellow (PY3 "Chrysler Yellow"). Surface primer: Tamiya or equivalent.