Curbside Classic: Mack RM-Series 4×4 – The Really Tough Mack

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(first posted 2/3/2014)    Mack’s legendary R-Series had a well-deserved rep for toughness. We praised them effusively here. But if one needed an ever tougher Mack, to take on all sorts extreme jobs off and on the road, there was the RM and DM (with offset cabs) Series.

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This AWD Mack RM that I found sitting along Hwy 58 in Oakridge has “Fire District” written on its door, and was rather likely used in some related task on the vast forest lands in the area, perhaps with a water tank on the back, or some other function.  Whatever it was, it must have involved rugged conditions, because this truck was built for that.

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Of course the cab is shared with the R-Series, but other than that, these DM and other related special-duty Macks were pretty different from the ground up. Their most distinguishing feature is the very old school free-standing radiator, not just a shell. This is the real thing, and that radiator cap is very much functional. The only thing missing here is of course the iconic bulldog sitting on the cap. Maybe they got tired of replacing it, since it unscrews so easily, unlike the ones fixed to the tilt-up fiberglass hoods of the R-Series.

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Let’s take a look at what goes into a 4×4 Mack RM, starting on the ground floor. It’s not like one has to get down very low to see what’s there, like this front axle. Beefy.

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Here’s the transfer case, just behind the cab. Power goes in on the upper level, in front, and then sent to the two axles on the lower level.

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The rear axle is supported by no less than three sets of leaf springs. Bet it rides a bit stiff without a load.

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Here it is from the front. Tough stuff, all of it.

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I should have popped open the hood, but there’s little doubt that it was anything but Mack’s turbocharged six-cylinder Maxydyne under there. From this side, I saw what I took to be the shift lever for the transmission, and some other control poking out of the that quadrant.

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But from this side, it was clear that there was no clutch pedal, and that the quadrant was for an automatic transmission.

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Sure enough; the control for an Allison five speed automatic, no doubt. Allison, formerly a GM division, developed the world’s first heavy-duty automatics near the end of WW2 already, probably based on Hydramatic principles. They made units to work in tanks as well as the Budd diesel railcar. And of course, the Allisons were (and still are) the go-to choice for automatic truck transmissions.

I drove Ford Super Duty dump trucks and cement mixers for Metro Pavers in Iowa City in the summer of 1974, and their whole fleet was Allison-equipped. It was my first exposure to them, and they certainly made the job…less challenging; or a bit more boring, as there was no opportunity to keep my clutchless-shifting skills up to snuff. The Allisons shifted mighty firmly, not unlike an old Hydramatic under full throttle. Nobody would call it a slush-box.

I’m still a bit perplexed about the big lever coming up from the floor. It’s probably an auxiliary transmission, presumably only shifted while in neutral, for low range(s) suitable for some serious off-roading.

While five speeds for the main transmission might sound like too few, the Mack’s engine was unusual in having a much wider torque band than average, and was often paired with a five-speed manual too, without getting bogged down.

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The RM/DM Series trucks were essentially all custom trucks, tailored to the specific job expected of them. Our featured truck is about as small as a RM as one is likely to find, with only a single rear axle. Most are at least tandems, and used in heavy haulage of all sorts, including prime mover duty for pulling really heavy loads. Over its axle sits ballast, so as to not spin its tires on take-off. This one here is pulling a 140 ton load, but that’s on the light side for serious over-the road heavy haulage.

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If you’re from New York or New Jersey, RM/DM-Series Macks may be familiar to you, as they seem to be particularly popular, if not dominant, for the construction, concrete mixers and other hauling jobs there. They were everywhere in NYC back when I used to visit regularly, and in the early morning hours they could be seen hauling big garbage dumpsters and such. And they were all painted that same dark shade of green. And of course the company names painted on them were all Italian. The ultimate Mafia-mobile. Maybe Mack had a special deal with them that they couldn’t refuse?

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Here’s one with a few sticks on its back. Wherever there was a tough job to do, there was a Mack willing to take it on.

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Yes, this twin-axle RM truck is looking downright petite. Maybe it’s going to get a pickup bed mounted so someone can show up all those jacked-up diesel Ford F-350 4x4s.