The diesel engine enjoys the most versatility and long-term durability of any internal combustion engine ever produced. Its do-everything capability (and steadfast reliability while doing it) makes it the ideal choice for virtually any engine swap project. Whether it’s intended to power a trail-ready Jeep Wrangler, a fuel-efficient daily driver, a high-horsepower competition vehicle, or a heavy-duty tow-rig, diesel often gets the nod over gasoline, and rightfully so. But what are the keys to pulling off a successful diesel swap? The list is long and, if you’ve never performed one before, more extensive than you think.
From sourcing an engine to picking the right transmission, and from choosing the right supporting diesel engine parts and components such as motor mounts, adapter plates, and wiring, to deciding which suspension and axles to run, the diesel swap journey is much more than simply dropping an engine into place. This time, we’re highlighting the fundamental pieces in the diesel swap puzzle. We’ll even cover the little things you hadn’t considered, like the proper cooling system, emissions compliance, and the invaluable benefits of having a donor vehicle at your disposal. Our list of diesel swap essentials is designed to make your Cummins conversion experience as painless as possible.
1. DIY Vs. Outsourcing
Before a single wrench gets turned and, even more importantly, prior to obtaining your 
swap candidate, it pays to evaluate your own skills. The average diesel swap calls for sound mechanical, wiring, and fabrication know-how—and no matter what, at some point in every diesel swap there is a need for a custom or one-off part to be made in order to make something work (think custom-sized radiators, intercoolers, overflow tanks, and even an angle change on a downpipe). Outsourcing either of these skillsets can quickly blow tight, DIY budgets out of the water. Be honest with yourself and your capabilities at the outset and adjust your budget accordingly.
Larger Budgets & Man Hours
It pays to know that a bare bones diesel swap can easily run you $10,000 or more, even at a shop that’s familiar with the process. A basic conversion—such as a Cummins 6BT (with very little in the way of wiring) and NV4500 transmission combination—can call for 80 to 120 hours of labor for someone with prior swap experience. You’ll likely need to invest at least twice that much time if you’re a first-time swapper. Outsourcing the inevitable wiring and/or fabrication tasks is common in the DIY diesel swap game and, unless you’re familiar with electronics or welding, is highly recommended.
2. Don’t Reinvent The Wheel—Choose A Commonly-Used Engine
Especially in the case of novice or first-time swappers, it’s important not to bite off more
than you can chew. It’s a fact that projects get immeasurably more complex when you’re resigned to using a rare or seldom-used engine. Transmissions, supporting parts, and OEM or aftermarket support will be hard to find, and you could quickly paint yourself into a corner. Deciding on a common engine with high production volume and readily available factory (or aftermarket) parts such as a 4BT Cummins or 6BT Cummins, brings an endless number of options to the table. Transmission selection, adapter plates, integration components, and even engine rebuild kits will be all the easier to source.
If You Have To Reinvent The Wheel
If you’re dead-set on swapping a rare, exotic, or off-the-wall diesel engine into place—which is the entire point behind some builds—make sure you’ve got plenty of budget or the right skillsets to accomplish your goal. Take Kevin Tetz’s Pro Touring Chevy C10 build for example. The host of Hands On Cars and producer of Paintucation chose a core 5.0L Cummins V-8 out of a ’16 Nissan Titan XD to power his classic Chevy, and in order to make everything work enlisted the help of the late Joshua Chapman of CFT Performance. Then, relying on his own skills, Kevin reskinned the ’71 C10 as a ’67 to satisfy his own personal taste.
3. Rebuild The Engine You Choose
No matter which power plant you choose, we highly recommend tearing it down and
rebuilding it—or having a professional familiar with your specific engine do it for you. Take-out engines aren’t guaranteed to last 100,000 miles, or even an hour—they are used, often have an unknown service record, and need to be treated as such. If don’t know the internal health of your engine, you may never have peace of mind when driving the finished project. In the end, the entire point behind most diesel swaps is to install a forever engine, something that never leaves the frame rails again. Cover your back. Start fresh.
Reseal The Engine You Choose
If a complete rebuild is out of the question, at bare minimum treat your second-hand engine to a reseal. Not only will this give you a glimpse inside the engine, but it will rule out pesky oil leaks from surfacing once it’s installed in the finished vehicle. If a reseal isn’t in the cards, at the very least perform a compression test as well as a blow by test before you trust the engine to long-term reliability. It can’t be understated that you need to have some idea as to the health of your used engine. Diesel power plants often lead very harsh lives, and high load, high miles and considerable operating hours are often on the clock by the time we get ahold of them.
4. Get Accurate Measurements Of The Engine
Break out the tape measure... You need to know the length, width, and height of your fully
dressed engine before any form of integration begins. This includes the turbocharger hanging off the exhaust manifold being included in the overall width or height (depending on its location), and the front drive accessories being factored into the front-to-back length. For tighter engine bays, the packaging size of the 4BT Cummins has long-been a good choice, as many Jeep Wrangler, Ford Ranger, old Toyota pickup, Bronco, and even Dodge Dakota owners can attest to. The packaging size of a V-6 (and even smaller V-8’s) makes them suitable for smaller engine bays as well.
Obtain Accurate Measurements On The Swap Vehicle
It goes without saying that you’ll have to compare the engine’s measurements to the space available in your swap vehicle. Of vital importance is confirming how much space exists between the frame rails, if the engine can clear the firewall, and whether or not you’ll be able to close the hood… Solutions for these types of predicaments are often solved through the use of modifying (widening) the frame rails, moving the firewall back, and running a cowl hood (or even adding a body lift). Here, it’s imperative that you remain realistic with your plans and your abilities. Swaps that entail shoehorning a 6BT Cummins into an old-school Power Wagon (shown) require a lot of custom work and finagling vs. what it takes to install a 4BT.
5. Pick The Best Engine For Your Needs
No matter what your goal is—be it for maximum horsepower, million-mile durability, peak
fuel economy, utmost simplicity, trail-conquering off-roadability, or the quintessential tow-rig—choose the engine that will best allow you to get there. Example: don’t choose a 4BT to repower a full-size or ¾-ton truck you plan to tow 10,000 pounds (or more) with. A larger, inline-six or V-8 diesel will be a much better candidate for the latter application. For lighter towing, commuting, a trail rig, or even a vehicle built to run on alternative fuels, the old, mechanical injection 4BT will suffice, and then some. Parting tip: whether your swap is a DIY project or a shop is handling it for you, source the engine yourself. You don’t need to pay a company to hunt down a diesel engine for you, unless you really want to.
Choose A Readily Available, All-Around Performer
For vehicles intended to be used for a myriad of tasks, you can’t go wrong with the 6BT Cummins. This old-school, all-iron, 2 valve per cylinder inline-six is legendary for reliability, low-end torque, horsepower potential, and simplicity. Benefitting from 100-percent mechanical injection, the 6BT (like the 4BT) only needs a starter enable wire and a functioning fuel shut-off solenoid to run. Thanks to its high production volume, it’s highly abundant, enjoys reasonably affordable parts, and benefits from all kinds of aftermarket support. In the performance world, the 6BT can be made to turn out as much as 1,500 hp in competition use, but can also be configured to produce 500 hp for reliable daily driving and towing use.
Additional, Jack-Of-All-Trades Type Engines
Other popular, readily available diesels with promising performance potential include
GM’s 6.6L Duramax, the Navistar-built 7.3L Power Stroke, and Ford’s 6.7L Power Stroke. They may not be as affordable as a 6BT Cummins to rebuild, but they offer solid reliability and sound drivability characteristics, even when highly modified. They are electronically controlled, but a donor vehicle—complete with all of the original wiring and modules—makes them highly feasible swap candidates. Junkyards, Facebook Marketplace, and complete, aftermarket long-blocks make obtaining either V-8 oil-burner a relatively easy process.
6. Motor Mounts
When it comes to mounting the engine, there is a lot more to a pair of motor mounts than initially meets the eye. Diesels create an inordinate amount of NVH (noise, vibration, and harshness), especially the 4BT and other compression-ignition engines that aren’t inherently balanced, so picking the right motor mounts is key in the NVH dampening process. In our experience, hydraulic motor mounts do the best job of isolating vibration and engine noise from the chassis. However, we’ve also learned that the exact location of the motor mounts is significantly more important than the specific type of motor mount.
Location, Location, Location
As stated above, the location of the motor mounts has more to do with quelling NVH than
the type of mount. To effectively dampen vibration in a 4BT swap for example, the motor mounts should be installed at roughly a 45-degree angle, and mounted above the center line of the crankshaft. For even weight distribution, the motor mounts should also be centered on the block. On certain model year 4BT engines, it pays to be mindful of the location of the vacuum pump/power steering pump combo or the motor mounts won’t clear. Close attention should also be paid to the clearance near the starter…
7. Choose A Common Transmission
Again, don’t reinvent the wheel. Things can get complicated quickly if you try to pair a high-tech, electronically controlled transmission with a diesel engine it was never intended to bolt up to. A great example of this would be attempting to get a late-model Ford Super Duty powertrain control moduel (PCM) to communicate with an Allison A1000 transmission control module (TCM), which is anything but a cut-and-dried undertaking. Throw an electronically controlled Cummins into the mix and you’ll have the Cummins ECM to worry about, too… This is why selecting an engine and transmission combination that was used in an OEM application is a great way to avoid complexity, ultimately easing the entire integration process.
Common Transmission Examples
In addition to running a factory engine-transmission pairing, you’ll have OEM powertrain
ZF 2004 S6-650 (ML6) Six Speed RWD Manual Transmission for GMC 2500 HD Sierra, GMC 3500 Sierra, Chevrolet Silverado.
matching on your side. So not only will the two components bolt up without a hitch, but you’ll have a combination that was actually designed to work together flawlessly. Behind the 6BT Cummins, the NV4500, 46RH, 47RH, 47RE, A518, and Getrag 360 will integrate seamlessly. Behind the 4BT, the NV4500, AX-15, TH400, SM465, and BorgWarner T19 make for great transmission options. In 6.6L Duramax applications, the Allison A1000, 4L80E or ZF-6 make the most sense. For 7.3L Power Stroke swaps, either the E4OD, 4R100, ZF-5, or ZF-6 can be used with ease. In 6.0L Power Stroke applications, the 5R110W and ZF-6 will work with factory-sourced components, and for 6.7L Power Strokes a new age 6R140 TorqShift automatic makes the most sense.
8. Adapter Plates
It goes without saying that engine-to-transmission adaptation is important in any diesel swap. In fact, without hunting down the proper adapter plate and flex plate (if automatic) or clutch (if manual), you won’t have a diesel swap at all. And when an adapter plate can’t be sourced, a custom unit has to be made, which brings us to our next point. It’s vital to remember that, when you’re drawing up plans for a custom adapter (or looking at aftermarket units), you keep in mind which starter you plan to run, as well as its mounting location.
In Addition To The Adapter Plate…
It pays to weigh your options in flex plates (for automatic transmission applications). If
your horsepower goals go beyond the factory rating, it’s wise to consider an aftermarket, custom billet replacement, or cryogenically treating an OEM unit for improved strength. Another key component in any diesel swap with an automatic transmission exists in torque converter selection. If possible, the converter’s stall speed should be matched to the engine’s turbocharger for proper drivability. In manual transmission diesel swaps, a good clutch (via aftermarket or OEM) and slave cylinder will need to be secured.
9. Axles
Diesels produce gobs of low-rpm torque, so making sure you’ve got axles that can handle plenty of grunt is paramount. If sourcing larger, burlier axles for your project is out of the question, there are ways of beefing up your existing running gear. This typically includes stronger, higher quality material (and increased spline count) axle shafts, locking differentials, cryogenically treated internals, or even an axle truss. And while axle reinforcements will add cost to your project, a diesel engine’s abundance of low-end torque and its appreciation for being under load means you don’t have to run low ring and pinion gearing (high numeric). This also works in your favor in the fuel economy department.
Axles Fit For Diesel Duty
As you can imagine, axle selection will vary from swap to swap and from intended
application to intended application. But what we can tell you, unequivocally, is that the following front axles are frequently employed in ¾-ton and larger diesel truck swaps with tremendous success: the Dana 44, AAM 925, and especially the Dana 60 solid front axles. Common and proven rear axle choices include the Dana 70, Dana 80, and AAM 1150. Either of the above options can be found relatively easily, and each unit enjoys plenty of aftermarket support for those looking to reinforce them.
10. Suspension
First and foremost, diesel engines are heavy given their size—and they’re notably heavier than the gasoline engines they often replace. Front-end wear-and-tear, along with ride comfort and handling should be a concern with any diesel swap. Although we’ve seen the factory suspension on late model Ram 1500’s, fitted with 5.9L and 6.7L Cummins engines, work in commuter and drag race applications, we can’t say what long-term durability and overall handling looked like a few years down the road. In many off-road builds, front axle to oil pan clearance is a major consideration that must be taken into account as well.
Donor Suspension Systems Are A Good Starting Point
This is especially true in budget builds, although a donor vehicle simplifies virtually any
diesel swap, regardless of budget. Provided you’re not piecing together a one-off off-roader, electing to go with the factory suspension that accommodated the axle(s) of your choice is a wise starting point. Repurposing the leaf springs parked above the rear Dana 80 in your 6BT-equipped Dodge Ram 3500 donor, along with the OEM coil springs used up front over the Dana 60, makes for a proven, direct swap suspension arrangement right out of the gate. After all, those coil springs were engineered to last indefinitely under the weight of a 1,000-pound Cummins.
11. Wiring
Wiring. It’s the most tedious and often-dreaded part of the diesel swap process, and it stumps many DIY’ers. Depending on the vehicle and powertrain you decide on, wiring everything yourself could call for anywhere from 25 to 40 hours or more of involvement. Even when properly integrating a mechanically injected Cummins into a late-model, newer generation Ram, you’ll have to strip down the factory wiring harness and figure out each of the key-ons. You’ll also have to weed through all the pin-outs and find out what you do and don’t need to make the engine and/or accessories work. And even then, you may have to configure work-arounds to make the truck drivable, such as tricking the computer into believing the factory transmission is still in place.
Plug-And-Play
If you’re not the wiring type, and a plug-and-play engine harness fits your budget, don’t
hesitate. It will save you hours upon hours of labor time. A handful of reputable companies can rework and transform a factory engine harness into stand-alone condition while also maintaining that clean, OEM look. Additionally, complete stand-alone ECU’s and wiring harnesses are available from Bosch and MoTeC and do a great job of controlling and fine-tuning today’s electronically controlled, high-pressure common-rail diesel engines (and transmissions). For engines with few electronics to speak of, such as in a P-pumped 5.9L or 4BT Cummins swap, the wiring task is much more simplified for the DIY’er or shop tasked with sorting through the harness.
12. Proper Cooling (Via Radiator)
Adequate cooling is a top priority for any engine, but it’s even more important in a diesel swap application that will be used to tow. To achieve proper cooling, the radiator has to be sized appropriately. The best way to ensure this is the case is by retaining and reusing the original radiator mounted in front of the engine you’ve chosen (again, the importance of having a donor vehicle at your disposal can’t be overstated). Going this route means less reengineering of the fan shroud, radiator hoses, and overflow reservoir. If space is tight under the hood, you may have no other choice than to have a custom radiator made.
Proper Cooling (Via Fan)
Efficiently moving air across the radiator is key, which means the correct fan (and fan
shroud) is of utmost importance. Race use engines or low-speed, off-road applications (where little airflow is passing through the radiator) can benefit tremendously from electric fans, and they are often the best solution for ultra-tight engine compartments. However, in towing applications mechanical clutch fans have proven the more effective and durable choice. In applications where space allows it, upsizing the diameter of the fan itself can yield a major improvement in cooling ability. One popular combination joins a ’94.5-’97 7.3L Power Stroke mechanical fan clutch with a 23.25-inch diameter fan from a 6.0L Power Stroke. We’ve seen this arrangement on countless common-rail Cummins swaps, where it’s made possible thanks to the use of a Cummins fan hub adapter.
13. Intercooler
To intercool or not to intercool. Unless space rules out the use of this particular heat 
exchanger, always make sure your turbo diesel is equipped with one. Be it air-to-air or of the air-to-water variety, an intercooler brings considerable long-term durability to the table, namely in the form of reducing intake air temps (IAT’s) and exhaust gas temperature (EGT). This makes life easier for both the engine (namely the pistons) and turbocharger. For most applications, a traditional air-to-air unit will suffice. This type of intercooler is the simplest to install and the most commonly used. They can be mounted in front of the radiator, or even located remotely behind the front bumper.
Air-To-Water Intercooler
Although a bit more complex than an air-to-air unit, an air-to-water intercooler can’t be overlooked for ultimate cooling efficiency. This intercooler uses engine coolant or water to reduce intake air temperature, thereby cooling EGT. There are more failure points in one of these systems, but in applications where space is limited, they are a sound, highly efficient solution. The air-to-water core consumes much less real estate than a traditional air-to-air intercooler does, and it can be mounted anywhere in the vehicle (even in the cab!). Denser intake temps equate to more horsepower, which explains why this style is so common in diesel motorsports.
14. Transmission Tunnel
Big, burly transmissions are often required behind big, torquey diesel engines. The only problem with this is that, depending on vehicle application, there may be a lot of fabrication work in store to make the transmission fit. For off-road and lowered diesel swaps alike, the transmission may need to be located up, off the ground as high as possible for maximum ground clearance. This means more of the transmission will protrude into the cabin, and that extensive tunnel work will have to be performed. The best advice we can offer here is to leave at least ½-inch of clearance around the transmission case on every side.
Get Accurate Measurements
Like the engine, a few extremely vital measurements need to be taken before your diesel-rated transmission can be shoehorned into place. First, you’ll need to know what your bell housing clearance is at the vehicle’s firewall. Then you’ll want to make sure you have proper clearance around the starter (and maybe even possibly relocate it). And finally, you’ll have to find out if there is ample space for the transfer case you plan to run, should the swap project be four-wheel drive. Custom driveshafts and aftermarket transfer cases are always an option, but sticking with a complete combination out of a donor vehicle is almost always the easiest way to integrate a transmission and transfer case.
15. Know Your Local Emission Laws
There is no point in performing a diesel swap on a street-driven vehicle if you won’t be able to register and legally drive it on the road once it’s finished—and Big Brother has the final say. This is why it’s imperative that you familiarize yourself with your local diesel emissions regulations. They can vary significantly from state-to-state, and even county-to-county. Do your homework first, and get it done before you secure the engine that will power your project. Also, and if your local emissions laws require it, make sure any performance parts you install on the engine are CARB-certified (complete with an E.O. number) or are 49-state legal if you reside outside of California. You don’t want to go through all the labor, money, and time involved in your dream diesel swap only to have it fail a smog test or vehicle inspection.
One Last Time: Obtain A Donor Vehicle!
Not to sound like a broken record but, if at all possible, source an entire donor vehicle at the outset of your project. Trust us, this will save you countless trips to the salvage yard, from having to buy parts off of Marketplace, and from ordering parts direct from a dealer or online retailer in order to make something work. With the right donor vehicle, you can get your hands on the engine, transmission, wiring, ECM, TCM, and sensors you’ll need to get the project running and drivable. Additionally, you’ll have diesel-rated axles, suspension, and even a heavy-duty frame at your disposal. A donor vehicle can be the ultimate contributor in any diesel swap.
