Caption: New Zealand-based Caterpillar D8H-46A series of the Ministry of Works on a hydro project “somewhere in NZ”. This tractor is equipped with a cable-operated 8S blade and No.8 rear radial arc ripper. The PCU for the blade was a No.24, mounted on the front of the tractor.
This is an early 235hp machine as evidenced by the smaller diameter exhaust pipe. (Pic: Richard Campbell).
Following on from the September Contractor Classic Machines article on the early Caterpillar D8s, we look at subsequent developments of the machine – tracing the period 1955 through 1982, and encompassing all the D342-engined variants of the machine. By Richard Campbell.
By 1955, Caterpillar realised that the D13000 diesel engine, employed in previous versions of the Caterpillar D8, had reached the end of its development potential and it could not wring out any more horsepower. Fortunately, Caterpillar had been working on a replacement for the D13000 engine, known as the D342. This was a six-cylinder, inline, non-turbocharged engine with an output of 191 flywheel horsepower.
The new engine was fitted to two new versions of the D8 tractor, which had been under development. First was the D8E-14A series machine that had a five-speed direct drive transmission. The second was the D8D-15A that had (for the first time in a D8 tractor), a three-speed torque converter drive transmission. As with previous iterations of the D8, both versions were 78-inch gauge and were simultaneously released for sale in 1955.
Now, here’s where things become a little confusing. Starting in 1956, Caterpillar introduced two more versions of the same tractor, this time with oil-cooled steering clutches and brakes – replacing the former models’ air cooled items. Although they were still 14A and 15A series machines, they had a new designator, the D8E- 14A becoming the D8F (above s/n 14A3861) and the D8D-15A becoming the D8G (above s/n 15A1673).
Prior to the introduction of these two tractors, Caterpillar had simply referred to the model D8 tractor as the ‘D8’ (1H, 8R, 2U etc.), leaving any defining model numbers such as A, B, or C off the designation. The D8-14A/15A was manufactured only in the USA, spanning the period 1955 to 1958 with over 8000 14A models and 3600 15A models being built. A bare D8-14A weighed approximately 18.5 tons and a D8-15A, around 19 tons.
In response to customer requests for an even larger, more powerful, and heavier D8 tractor, Caterpillar engineers came up with the model D8H, introduced in 1958. The D8H was a complete redesign featuring a wider 84” gauge, a solid equaliser bar (all previous versions had used multiple spring type equalizer bars), hydraulically actuated steering clutches, and a turbocharged version of the Cat D342T engine, initially rated at 225 flywheel horsepower.
The first model released for sale was the D8H-36A series, which was a direct drive machine, and D8H-35A series for the torque converter drive model. Also, in another first for the D8, a powershift transmission version was offered.
This machine was introduced in 1959 and was known as the D8H-46A series, and featured a three-speed forward and reverse powershift transmission manufactured by Caterpillar.
In 1961, the D342T engines output was raised to 235 flywheel horsepower for all versions of the tractor. Also, for the first time, due to the demand for, and popularity, of the machine Caterpillar manufactured the D8H outside of the USA at the old Birtley plant in the UK.
Tractors made at this facility bore the serial number prefix 22A (for the direct drive model), and 52A (for the torque converter drive model). The powershift version was also manufactured at the Birtley facility and was known as the D8H-68A series, with some 4300 powershift drive machines being built there.
A further horsepower increase was made in the mid-1960s, when the output was raised to 275 flywheel horsepower. This horsepower increase was also made to UK-manufactured product as well. The D8H could be found in all manner of applications, from mine, quarry, general earthmoving and forestry, and performed admirably in all of them.
As an indication of the powershift D8H’s popularity, Caterpillar USA produced over 33,000 of them before they were replaced by another improved version, the D8K. Bare weights: D8H-35A 20.5 tons; D8H-36A 21 tons; D8H-46A 22 tons.
By the early 1970s, the D8H was in danger of being overtaken by more modern products, notably from Japan. In order to remain competitive, a further serious upgrade of the D8H was undertaken, resulting in the model D8K, introduced in 1974.
Improvements included: pilot operated hydraulic controls; sealed and lubricated track chains; a pinned equaliser bar; and combined braking and steering controls. The D342T engine was also raised in horsepower from 275 to 300 flywheel horsepower.
Unfortunately, the increase in horsepower was not well tolerated by the D342T engine, which had a habit of dropping cylinder liners. Cat engineers partially solved the problem by fitting a spacer plate between the cylinder block and cylinder head, but like the pre-1945 D13000 powered D8s, the D342T engine was now at the upper level of its development potential. Still, the spacer plate engine seemed to work quite well if it was respected.
There were two different versions of the D8K manufactured in the USA, the direct drive D8K-76V series and powershift transmission D8K-77V series. Total production of US assembled D8Ks exceeded 6000 machines.
Production was also undertaken in the UK of the powershift transmission D8K, a version known as D8K-66V series.
The Caterpillar D8K was in continuous production from when it was first introduced in 1974 right through to 1982 when it was replaced by the completely new D8L model, which featured Caterpillar’s ‘Hi-Drive’ track system. (The D8L falls outside of the scope of this article but, will be covered in a future issue of Contractor). The bare weight of a D8K-77V series tractor was approximately 30 tons.
Due to the robustness of the basic design, thousands of conventional track drive D8s are still in operation throughout the world and, provided spare parts are still available, look like being so for many years to come.
Mention must also be made at this point of dedicated pipelayer versions of the D8 tractor, the models 583C, 583H and 583K.
However, these specialised machines fall outside the scope of this article.
For the model collector
The pickings are pretty slim here, as no models currently exist of the D8D/E/F/G series machines (your author built his own one!).
As for the D8H, apart from a hideously expensive 1:24 scale model by CCM (over US$3000) and a very small 1:160 scale version in pewter, no other models of the D8H are available other than a 1970s-era kitset made by AMT (that you must put together yourself ).
A fine looking model can be made from this kit with time and patience.
The D8K on the other hand is available in a limited run 1:48 scale model by CCM in three different configurations – U-Blade with 3-shank ripper and ROPS cab, S-Blade with single shank ripper and ROPS cab, and a version with an angle blade, winch, and open ROPS.
There is also a pseudo-D8K by Arpra, but this has hideous tracks and is based on Conrad’s former D9G model, so is not really a serious contender.