Home » Lightweight Chrysler Slant Six Vs. Heavy Cummins Diesel: Which Of These RVs Would Last Longer?

Lightweight Chrysler Slant Six Vs. Heavy Cummins Diesel: Which Of These RVs Would Last Longer?

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The Autopian has been sorta-shopping for an RV for quite a while now, and part of the reason we haven’t pulled the trigger is that we’re looking for something special. Something stylish, vintage, reliable, spacious, and most importantly: something with a stickshift. It may come as no surprise to many of you that most RVs are automatic; America gave up driving the stickshift many decades ago. Still, if you look hard enough, you will find manual transmission RVs, and these two that I’m about to show you are the most compelling in my eyes because those standard transmissions are hooked to unkillable motors.

The one thing I care about when it comes to an RV is that it works. I’d rank “removing and rebuilding an RV’s engine or automatic transmission” somewhere between “debate with Tesla YouTubers whether the Model Y is the greatest car of all time” and “rebuild an automatic transmission valve body works” on the list of activities I want to do anytime soon. As such, the only RVs that remotely intrigue me are ones with manual transmissions (which tend to be easier to remove and way easier to rebuild than autos) hooked to unstoppable motors.

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Recently, two RVs have popped up for sale in the LA area, and not only are they both relatively affordable, but they both feature engines known for Never Dying. The first is a smaller RV called the Clark Cortez:

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This is a machine developed by a Michigan-based company named Clark, which is known for its forklifts, and also for pioneering those tractor-like tugs you see at airports. The Cortez motorhome is a compact front-wheel drive cruiser outfitted with a standard 225 cubic-inch Chrysler slant-six mated to Clark’s own four-speed manual transmission. And it’s gated!:


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My colleague Mercedes Streeter has already written the full story of the Clark Cortez, so read that if you have a chance. Here’s a quote describing the vehicle’s basic layout:

According to Clark, 60 percent of the camper’s weight rides on the front axle for handling, traction, and stability. In terms of motorhome equipment, everything that you’d expect to be there is present. It sleeps four adults with a floorplan that includes a full kitchen and bathroom with shower and toilet. The coach’s engine feeds from a 25-gallon fuel tank and there’s storage for 30 gallons of water onboard. The brochure doesn’t note anything about waste capacities.


I’m a huge fan of the Chrysler Slant-Six. I daily-drove a 1965 Plymouth Valiant for an entire winter in Michigan, and I never had any issues with the Leaning Tower of Power, despite me having bought that old rustbucket for just $2 grand. The motor ran like a top!

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If you don’t believe me, give this a watch!:

So I’m already sold on the Clark, even if I know it’s got more weight to carry around. The Cortez weighs 7,000 pounds, or over twice what my Valiant did (it helped that my Valiant had undergone some salt-induced weight reduction).

Anyway, here are some more photos of the Clark from the Facebook listing. The thing needs work — with rust on the roof, insulation missing, and a dashboard that looks like it was submerged in the ocean for a bit — but at $5,800, maybe it’s still an OK deal?

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Still, even if that Cortez isn’t perfect, its trump card is the Chrysler slant-six. It’s a truly unkillable motor, and the prospect of that being hooked to a beefy four-speed built by a company known for its industrial machinery — I like it. You know what else I like, though? Like Clark, it’s a company that starts with a C. It’s also based out of the midwest, and it’s also in the heavy machinery business. I also spent a summer there working as an Advanced Concepts Engineering Mechanical Development intern.

I’m talking about Cummins.

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That’s what you’ll find in this 1971 Grand Coach Crown RV conversion (clearly, this was once a bus). The engine is a Cummins 220 “pancake motor.” It’s mid-mounted, and hooked to a Spicer five-speed (some had Eaton Fuller 10-speeds; the seller on this one says it’s a five-speed) with a huge stick wrapped in a hilariously tubular boot:


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And the living quarters appear to be in decent shape, too:

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What do I know about the Cummins NHHS 220? Not a ton; it’s 12-liters of fury compared to the Clark’s 3.7, and it makes over 600 lb-ft of torque, which is a good thing, because this camper is humongous.

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It’s a fascinating motor — six cylinders sitting on their sides, all mounted in the center of the coach. This big-rig motor is unstoppable according to the commenters on Barn Finds, with HotPotato writing:

The Crown Supercoach is indeed iconic…and not just for the retro-future mega-Twinkie styling which was unchanged for 40+ years. Most school buses are built on medium-duty lowest-bidder beer-truck chassis. Crown built its own chassis, and every mechanical component installed was from the world of 18-wheelers. That’s the reason for the mid engine and the tall floor: there’s literally a semi powertrain under there, cleverly turned sideways to save space. It can be serviced anywhere with standard 18-wheeler parts, and it will outlast any other school bus on the road: a million miles is nothing, and literally the only reason these bad boys were retired after decades of service was because emissions rules required it.

Here’s what commenter Dave Wright had to say::

They moved 80,000 GVW trucks all over the country even before freeways. Geomechs is on the money as usual…….220 Cummins built the prosperity we have today moving 80,000 of whatever wherever we needed it.

And here’s a quote from skoolie.net:

I would guess it’s most likely a Cummins 220 HP (non-turbo). If it runs well it’s a very fine engine and with
a 10 spd. behind it it’s very close to a perfect combination. The NHH-220 specs: 743 cubic inch 220 hp @ 2100 rpm,
606 lbf/ft TQ @ 1600 rpm. The key is that the engine only produces about 600 ft/lbs of torque and the transmission
is built to take 900 ft/lbs and pull 80k lb. trucks all day long. You just can’t stress that transmission enough with that
engine or fully loaded Crown to cause any real heat build up.

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Cummins itself writes about this “Pancake” motor on its website, noting that in 1954 it powered America’s very first diesel school bus:


The Crown Supercoach was the first diesel-powered school bus, with a Cummins NHH-200 hp horizontal ‘pancake’ engine mounted midship underfloor. The 12.2-liter diesel was ideal for hill-climbing and enabled the chassis to add a third axle, increasing to 91 seat capacity for school districts with growing student populations. Known as the ‘royalty’ of school buses due to exceptional build quality, the 40 ft. long Crown featured 26 roll bars in the roof for added strength and safety.

Clearly, these buses were beasts. But they also weighed at least 20,000 pounds.

So which of these — the lightweight slant-six “Leaning Tower Of Power”-equipped Clark mated to a four-speed or this huge Cummins-powered Crown hooked to a five-speed — do you think would be easier to maintain and last longer?

Topshot: Facebook Marketplace/Cummins/Plumkrazygarage

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Mercedes Streeter
Mercedes Streeter
16 days ago

The Crown Supercoach is a part of the Autopian RV/Bus Expanded Universe, too!


If it were my money, I’d get the Crown. Those old buses were built to last and the conversion looks like it was done professionally. Let Sheryl work her magical cleaning powers and we could sleep in it almost right away.

On the other hand, the Clark was built to be a motorhome from the start. It was also built with an all-steel body for longevity. Unfortunately, this one hasn’t lived the best life. We’d have to gut that roof. Then, there’s the FWD powertrain. This isn’t the Oldsmobile UPP with a Chrysler engine, but a more bespoke setup with parts that’ll be hard to find. Ignoring all of that, the poor engine has a lot of steel to move.

Super Bonk 3000
Super Bonk 3000
14 days ago

Oh wow, nostalgia. My school district had a fleet of Crowns, some double-axled, and in order to graduate HS I needed X more units as I was an unrepentant truant so I took a diesel course through the local ROP (Regional Occupational Program I think) tearing down various busted big engines and putting them back together. We did a field trip to the district bus garage where they had full-time mechs who maintained and repaired the Crowns. Day of our visit, one of the pancakes had its pistons out for I think rings, wrist pin bushings, and rod bearings (this was 1979 so memory is a little foggy). Guy said it only took a few hours to get them out and back in thx to the engine orientation. Bus was back on the road the next morning. Super-easy to work on.

Get the Crown. That Clark is interesting but not in a good way. Terminal rot and a custom drivetrain is a nope.

15 days ago

It sounds like the Crown will last longer, but it’s going to need a big rig garage to maintain. So the literal answer to David’s two questions is that the Crown will last longer, but the Clark will be easier to maintain.

15 days ago

That Clark is a goner. Too much rust and the roof is trashed. That will not be an easy task to make it tolerable to be in long enough to travel or stay anywhere. Too bad, I really like the shorter Rvs like that for personal use.

Blame Mercedes for this, but I’d say the bus would be better for The Autopian. It’s got some serious geek-cred being a classic manual diesel bus. I’m assuming this would be used for things like covering racing events or EAA or stuff like that so a big bus would be fine. If this were for a family that wanted to do a lot of national park nature camping then I’d say no to that. The RV conversion looks pretty decent from the pics. If it runs, drives and doesn’t smell to bad of mold and pee then go for it.

Would love your thoughts, please comment.x