A North American Classic: the Mack B61

The Mack B Series was introduced in 1953 and endured until 1966 – some 126,745 trucks later. It was offered with 10 different petrol engines, from 4.8-litre with 107hp in the B20 to 11.6-litre with 232hp in the B70.

A wide range of diesels were also offered: from the B61 up, the END673 in-line six, displacing 673 cubic inches and the END864 V8 and, from the B73 up, the Cummins 14.0-litre, rated up to 335hp, were available.

The Mack B61 model is perhaps the best-known classic Mack truck and many survive to this day around the world, in restored and unrestored condition. They were available as fire trucks, school buses and in a variety of rigid truck and prime mover configurations. The total number of B61s produced was 47,459 – far more than any other B Series truck.

The 1962 Mack B61 that was advertised in November 2020 in trucksales.com.au classified pages was something of a mystery truck. It’s believed that it first went into service with Mildura-based McGlashans Transport as a 6x4 prime mover, with a quad-box transmission – more on that later – and a tandem-drive back end with ’camel-back’ suspension,.

The drive tandem was interesting, with a longer dimension between the drive axles than normal. This configuration was popular in Canada at the time, because a slightly wider tandem ‘spread’ allowed a half-ton more legal weight over the drive axles than with the more normal (around-50-inch) distance.

This particular B61 later moved north, where it moved houses around Queensland from site to site and was then sold into Western Australia.

Len Houlahan had the truck at the time it suffered a cracked block, then new owner Peter Lynch found a later model END711 block. Peter Lynch said the truck went really well after that. In its later working years, the B61 hauled a mobile ablution building in WA and always attracted attention from ‘patrons’, Peter Lynch told trucksales.

Subsequently the venerable Mack was purchased by Gary Nunn, the founder of what is today’s Nunn Bros Haulage. After some additional cosmetic work over its years in residence there, the venerable B61 found its way into new hands and then into our For Sale pages.

In Australia, Mack has been around since the first AC Model arrived in 1919, for bulk haulage by the Vacuum Oil Company in Sydney.

The American military influx in early 1942 introduced Australia to the heavier NR 6x4 Macks and after WWII, some NR and EH Macks found their niche in off-road applications such as hauling timber, cattle and heavy machinery in the more remote areas of Australia.

Pre- and post-War Mack END diesel engines used pre-chamber heads licensed from Lanova and engines of this era had exhaust manifolds branded ‘Mack Lanova’. The advantage of this system was that the same engine blocks could be used for petrol and diesel engines.

However, Mack engineers knew that they’d have to develop purpose-designed diesel engines for the 1950s and, in co-operation with Scania in Sweden, came up with direct-injection diesels, branded Thermodyne. These END673 engines were slotted into a brand new range of heavy trucks, with modern, curved-mudguard styling and integrated headlights: the B Series.

It was the arrival of the legendary B61 Model Mack that established the brand as a leader in reliability and good value for money, in the toughest conditions Australia had to offer. The B61 Model was released in 1953 and, by the early 1960s, had established an enviable reputation in Outback Australia. It continued in production until 1966, when it was replaced by the R Series.

English-made trucks had shown they could handle heavy road-train weights, but these trucks were very slow – typically geared to less than 30km/h. Northern Territory road-train operators were able to double their average road speeds by using B Models.

Initial B Series Macks were powered by Cummins 220 in-line six-cylinder diesel engines, but retained Mack’s five-speed main transmission, with attached Mack auxiliary gearbox.

A year later, Mack introduced the Thermodyne with 170hp and that later grew to 211hp. Late-model B Series trucks with the turbocharged ENDT 673 had about 220 horsepower and that was ‘big grunt’ back then. (It’s easy to forget that the later, 1966 R Series’ Maxidyne – the word’s first high-torque-rise diesel – was launched with only 237hp.)

Between 1953 and 1966, around 80 per cent of all Macks were powered by Thermodyne variants.

The weak link of the Thermodyne – and all competitive engines – was a relatively narrow band between maximum power and peak torque and that necessitated lots of gear ratios.

In the case of the END673E engine, maximum power was 180hp at 2100rpm and peak torque was 500lb.ft (680Nm) at 1400rpm. To put that into perspective, you can get that much torque and twice the horsepower in one of today’s big utes!

The 180hp/500lb.ft B61 was expected to pull 32 tonnes GCM regularly in highway service and even road-train weights in the Outback. A modern ute has a GCM around 6.5-9.0 tonnes.

Mack’s solution to the engine-output issue was deep-reduction final drive gearing – as low-speed as 11.8:1 – and a choice of Quadraplex, Triplex and Duplex married-box transmissions.

These constant-mesh boxes combined a five-speed main box with either a four-speed, a three-speed or a two-speed ‘compound’ auxiliary section, giving 20-speed, 15-speed and 10-speed ratios. A variation of the 20-speed was an 18-speed, in which you couldn’t use two of the compound gears in conjunction with fifth in the main box.

The B61 boxes were operated by two gear levers and compound-shifts – moving both levers simultaneously – were necessary when either box ‘ran out’ of available ratios, necessitating ratio changes in both gearboxes.

The Mack transmission had its two gear levers sitting side by side, poking up out of the cab floor. Some blokes fitted small, tapered gear knobs and bent the levers even closer, allowing a good operator to make some compound shifts using one hand to ‘palm’ both levers at the same time. However, many compound shifts required the use of both hands, with the right arm inside the steering wheel rim.

The advertised B61 had both levers bent and one was fitted with a ‘palming’ style gear knob.

Naturally, the Mack B61 didn’t have synchromesh, so the revs had to be spot-on in order to make a clean shift: a double de-clutch when going up in ratio in either main or joey and a blip of the throttle to harmonise speeds of the input and output shafts in the boxes when choosing a lower ratio.

Upshifts were normally done at maximum engine revs – 2100rpm – and downshifts at 1700rpm. Letting revs drop below that point risked losing momentum and a downshift to a ratio that then wasn’t low enough!

‘Neutralising’ both boxes was a complete no-no, as there was no way to engage a gear unless the truck was stopped.

After two-stick truck transmissions largely gave way to Mack Maxitorque, triple-countershaft boxes and twin-countershaft Fuller Roadrangers and Spicers – all of which had one lever control of the main box and knob-mounted switches for splitter and range change control – drivers thought they were in heaven!

Then came European all-synchromesh transmissions, followed by today’s fully-automated shifters. Old-timers can’t believe gear-changing has become so easy!

It’s great to see increasing interest in old trucks; particularly in those models that helped conquer Australia’s vast distances, transporting raw materials, produce and livestock to markets and bringing necessities to remote-area communities. The Mack B61 was certainly one of those vehicles.