Caterpillardual axle

The Caterpillar 651 scraper

This month we have a closer look at the largest two-axle, single-engined scraper that Caterpillar has produced so far. by RICHARD CAMPBELL

Designed for fleet use in really serious bulk earthmoving situations, the first Caterpillar 651 appeared in 1962 along with a host of other Caterpillar scrapers in its new “600” series.
Some of the new scrapers did not last long in the company’s catalogue, however the model 651 was a stayer and evolved over the years into a real brute of a machine while its competitors fell by the wayside.
One of the secrets of the machine’s longevity has been its simplicity and the ability to be remanufactured, often more than once, making it an extremely cost effective way of moving large amounts of material.
Due to its size, the 651 is ideally suited to larger projects where it will spend a lot of time.
Another mitigating factor is the availability of suitable push tractors, as a single engined scraper of this ravenous capacity needs a decent sized push tractor to maximise its potential.
In the US, 651s are more often than not tandem pushed by D10-sized tractors for optimum payload and cycle times.
Fleets of these machines can be seen on YouTube carving up the countryside in spectacular fashion, many wearing the battle scars of a long hard life.

Caterpillar 651 development

When it was originally introduced, the Caterpillar 651 was powered by a 450 flywheel horsepower Caterpillar D346T, 60° V8 diesel engine mated to a Caterpillar nine-speed, barrel type planetary powershift transmission.
Bowl capacity was 32 cubic yards struck, 44 cubic yards heaped and all operations were fully hydraulic.
Engine power was soon boosted to 500 flywheel horsepower when it became obvious that the machine was a little underpowered. These scrapers were known by Cat as the 33G series and weighed around 48.5 tons empty.
While industry acceptance of such a big machine was at first slow, this was not to last.

Cushion hitch added

Following extensive development and testing by Caterpillar on the model 631B scraper, the cushion hitch ride control system was made optional for the 641, 651 and 657 in 1967.
As this was a factory only modification, and could not be retrofitted to existing machines, those 651s equipped with the cushion hitch were given a new serial number prefix, 44M, to distinguish them from non-cushion hitch machines.
Apart from a hefty weight increase to just over 57 tons, all other specifications remained the same.

The 651B

The 651 was given an upgrade in 1968, becoming the 651B.
Engine horsepower was increased to 550 at the flywheel and the new 8-speed powershift transmission, which was being introduced across the entire Caterpillar motor scraper line, replaced the 9-speed powershift used previously.
ROPS mounting pads were also made standard and more tyre options were added.
This turned out to be a very popular scraper for Caterpillar with the vast majority of the machines manufactured still in service, either as scrapers or modified as water carts.
The 651B was in continuous production from 1968 up until 1984 when it was officially replaced by the all new 651E.

The 651E

First manufactured in 1983, the 651E had a raft of new features, not the least of which was a new engine, the 550 flywheel horsepower Caterpillar 3412TA.
This was a 65° V-12, turbocharged and after-cooled diesel with a whopping 1649 cubic inch displacement which was a little more environmentally friendly than the D346 previously fitted to all 651s.
While capacity of the machine remained the same, some fairly major redesign had taken place increasing the structural strength of the tractor, bowl and hitch and changing the machine’s appearance along the way.
Empty weight had now risen to 65.5 tons making it the heaviest of all 651 variants, but it could still zip along at just over 30mph with a full load on board.
Unfortunately, the 651E currently represents the last of the 651 line, Caterpillar not having announced a replacement when it upgraded the similar, but twin powered 657G in 2006.
So, effectively, production of the 651 has ceased for the time being leaving only the 631K and 621K scrapers in the single engine, open bowl category in Caterpillar’s catalogue.


Caterpillar did not have too much opposition at this end of the scraper size scale but competition did exist.
Principal competitors were the Euclid (later Terex) S-32, Michigan 410, Allis-Chalmers (later Fiat-Allis) 460 and MRS 250A.
All of the above were gone by 1985 leaving Caterpillar alone in the field.
Wabco did not offer a machine in this size category and neither did International Harvester.


Along with cabs, heaters and the like, there were other options to outfit the 651 to suit your needs.
Athey products offered a hydraulically operated 50 ton rear dump in place of the scraper bowl, Rome Tool & Plow could provide compaction equipment to replace tired scraper bowls, and both Southwest Manufacturing and Klein offered water tanker conversions.

The Kiwi connection

Two used Caterpillar 651Bs have been imported for use on an irrigation project in the lower South Island. At the time of writing these machines had yet to go into service.
Watch this space!

For the Diecast Model Collector

The pickings are really slim here unless you have very deep pockets and connections overseas but surprisingly there is a model of the 651 available.
It was limited to a production run of 50 items and was made in 1:50 scale by Black Rat Models in the UK.
The model represents an early production machine without cushion hitch and a European style cab.
It was based on measurements obtained from a French machine that is still operational and is very accurate and impressively large.
At well over $1300, it is a rare and highly collectable piece if you can find or afford one.
The author would love one to add to his fleet!

Brief Specifications – 1977 Caterpillar 651B

Engine: Caterpillar D346TA, turbocharged and aftercooled V8 diesel rated at 550 flywheel horsepower @ 1900 rpm

Transmission: Caterpillar 8-speed semi-auto planetary powershift transmission

Top speed: 34mph

Steering: Full hydraulic, 90° to left and right of centre

Turning circle: 53ʹ10ʺ

Brakes: Air operated S-cam expanding shoe

Capacity: 32 cubic yards struck, 44 cubic yards heaped

Length: 50ʹ 4ʺ

Width: 14ʹ 2ʺ

Height: 14ʹ 1ʺ

Op.weight: 62.1 tons (empty), 114.2 tons (loaded)

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