Cummins Engine Swap in a Square-Body Chevy: What to Know

I’ve been a fan of Four Wheeler for as long as I can remember. I used to grab any and all off-road magazines off the shelf and read them when I was a kid at the grocery store with my mom. You can only spend so much time looking at potato salad with your mom. Often I came home with several of the magazines back when there were a ton. I’m sad that most of those magazines are gone, but am glad that Four Wheeler is still here. I’ve never written in with a question, but thought I would today.

Related: 1974 Chevy C20: Why Buy a New Truck When You Can Build Something Better?

I’m contemplating building a new (to me) tow vehicle although this is still all hypothetical. Something that I can use to tow my trailer with off-road toys or a camper that won’t break a sweat with a load behind it, and something that I can take down sandy back roads or to the camp site without worrying about getting stuck. I’d really like something that can tow about 10,000 lbs or a bit more with a crew cab for space. I really love the Cummins engines, the 12-valve and 24-valve 6BT engines, but finding one of those old Dodge trucks that has any life left is hard these days, and the crew cab trucks are expensive or pretty rough. I also like the style of the old GM Square body trucks and think that might be the best place to start since they seem to exist as crew cab trucks for sale locally with some regularity.

So I think I’d like to build a Cummins-swapped square body crew cab but have a few questions. First, am I stuck finding a used Cummins or can a crate engine be sourced from somewhere? Secondly, what’s the best transmission to use? What about the transfer case? Do I need to find a 4×4 crew cab square body to start with, or can I start with a 2WD Crew Cab and convert it to 4×4? Do I need to box the frame? Any insight or ideas would certainly help.

Thanks, Mark Lyden

Four Wheeler Tech Editor Verne Simons’ Response

Mark, you are not alone in being sad at the loss of most of the many awesome off-road magazines, and you are also not alone when it comes to wanting to build a Cummins-powered square body 4×4. On both topics we can agree. We can’t help bring back the other magazines but will keep working hard to make Four Wheeler the best it can be. In regard to a Cummins-swapped square body Chevy, others have had the same idea, making the task a bit easier for you. In the end it’s hard to deny the classic styling of the Square Bodies and they are plentiful, which means there are many great sources for parts and upgrades.

In order to make this project happen you will either have to be good at both fabrication and at finding a deal on good used parts, or you’ll have to have some deep pockets to fund the build as the price of the components can add up quickly.

Selecting an Engine—New, Rebuilt, or Used?

To answer your question about sourcing an engine, our buddy Cooper at Diesel Power Products tells us there are a few new and several remanufactured crate Cummins engines available in the aftermarket. You can spend what you want on a new or remanufactured engine at a place like dieselpowerproducts.com or start the hunt for good used parts. In either case, you can choose the Cummins diesel best-suited to your needs—12-valve, 24-valve, Common Rail, or 6.7-liter are all available—but be warned: we have heard some pretty sad tales about folks buying junk used diesel engines.

Manual or Automatic?

For transmission choices you have a couple of decent options, but in the end you should select based on what route you want to take and what you want your truck to do. If you can source a used engine locally, it may even come with a transmission which can be serviced, rebuilt, or replaced. You can choose between one of the reliable and venerable five or six-speed manual transmissions or one of the heavy-duty automatic transmissions that can be mated to the Cummins blocks. In manual flavor, you can opt for a NV4500 five-speed, or a G56, ZF6, or NV5600 6-speed, all of which work behind the Cummins.

Related: NV4500 vs. TR-4050 Transmissions

As far as autos go, you can run a built 47RH, 47RE, 48RE, 68RFE, Ford 5R110, 6R140, or an Allison without any problem. We’d stick to a name you know in the aftermarket to source a transmission, like ATS, Suncoast, or Firepunk, assuming you don’t have a local transmission shop to build you one to suit.

Turning All Four Wheels

When it comes to what transfer case you should use, that again may depend on your parts source. Here, the path of least resistance would be to go with the transfer case that came from the factory with the transmission you are running. Another nearly bulletproof and safe option would be to source an NP205, which is likely the strongest transfer case commonly used in trucks like this. NP205s are available in driver drop (Ford) or passenger drop (GM) configurations to match the front axle you will use. 205s have lots of upgrades from places like Offroad Design and are pretty easy to rebuild. Speaking of axles, you’d probably be better off with a Dana 60 in front, either a passenger-drop kingpin-style from a GM or a driver-drop Dana 60 from a Ford (the latter being much more common).

Which Truck to Buy

That brings us to another part of your question: should you start with a two-wheel drive or a four-wheel drive square body GM truck? Again, the answer is that it depends largely on what you can find. Ideally, you could get ahold of a GM one-ton crew cab 4WD with a kingpin Chevy Dana 60 front axle and an NP205, but those trucks are rare to come across.

Finding a 2WD crew cab square body Chevy or GMC to start with is going to be much easier, and while not ideal, this should get the job done with enough modification. Be aware that, depending on what Cummins engine you find and what transmission and transfer case you use, that GM Dana 60 and NP205 might not make the most sense.

Upgrades for Durability

When it comes to boxing the frame on a square body Chevy it’s more than just a really good idea. The frames on these trucks, while beefy, can always use some additional strength, and some of the 4×4 frames of this era are known for having weak spots around the steering box mount. Luckily (and getting back to what we said above when talking about 2WD vs. 4WD) Rob Bonney Fabrication (RBF) has steering box outer reinforcement plates for 1973-1987 Chevy K-series, and frame boxing plates for most GM K Series Trucks and Suburbans built between 1967 and 1987, as well as Suburbans from between 1973 and 1991. RBF also has frame boxing plates for 1973-1987 C-30 trucks with plans to expand into C-20 and C-10 trucks soon.

The company designed these frame boxing plates specifically for trucks receiving Cummins swaps, although many can be modified to be used even if a Cummins swap isn’t the end goal. RBF also has a few other parts available for GM to Cummins swaps including motor mount kits, radiator and intercooler mounting parts, battery trays, and a high-mount alternator bracket kit.

We love the idea of this build and want to do one for ourselves too. You should end up with a stylish solid rig that will tow with the best of them and should last for decades.

Watch a Full Episode of Dirt Every Day! Diesel Jeep Underwater

Into diesel engines? Then you might like this episode of Dirt Every Day. Fred Williams’ 1997 Jeep Wrangler TJ, affectionately known as Tubesock, was in need of a new engine, and he wanted to recreate a crazy stunt from the ’50s by driving his Jeep underwater. To achieve this, Fred swaps in a new Cummins ISF2.8 diesel engine, bolts on big Maxxis tires, and sets about waterproofing the whole Jeep to survive under 12 feet of water. Will the little Jeep be a submarine or just an anchor? Sign up for a free trial to MotorTrend+ to watch every episode of Dirt Every Day!

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