What Should You Replace First in a Cummins Conversion?
Swapping your stock engine for a Cummins is a bold move—and a brilliant one if you’re chasing durability, torque, and serious power. But make no mistake, a Cummins swap isn’t just about yanking out the old motor and slotting in a new one. It’s a complex process that demands attention to detail and a clear plan of action, especially when it comes to choosing what to replace first.
Let’s explore which components should top your list and how the right Cummins conversion parts can make or break your build.
Start Strong with the Engine Health Check
Before anything else, assess the condition of your Cummins engine. Whether it’s a 5.9L 12-valve, 24-valve, or a newer 6.7L, do a thorough inspection. You want a solid foundation before building out the rest of your conversion.
Check compression levels, inspect the injectors, and ensure there’s no turbo lag or signs of head gasket wear. If the engine needs a rebuild, now is the time to do it—not after it’s bolted into your chassis.
Match the Muscle with the Right Transmission
A Cummins engine packs a serious punch, which your factory transmission probably isn’t designed to handle. This is why a transmission upgrade often follows right behind the engine on any conversion checklist.
Depending on your vehicle type and driving habits, you might opt for a heavy-duty manual like the NV4500 or a built 47RE automatic. Be sure your new transmission pairs well with your Cummins and your driving needs—whether that’s off-road crawling, highway hauling, or city cruising.
Mounting the Beast: Motor Mounts and Adapters
Once you’ve locked in the engine and transmission, you’ll need to figure out how to make them fit together in your existing vehicle. That’s where conversion-specific motor mounts and adapter plates come into play.
Since Cummins engines are notably bigger and heavier than many factory engines, custom motor mounts are a must. Adapter plates and custom flywheels or torque converters ensure proper alignment between the engine and transmission. High-quality Cummins conversion parts in this category will reduce stress on your drivetrain and make installation smoother.
Stay Cool Under Pressure: Upgrade the Cooling System
Cummins engines run hot—especially under load—so your stock cooling system likely won’t cut it. Upgrading your radiator is essential, preferably to a high-capacity aluminum unit.
You’ll also want to consider electric fans for improved airflow, heavy-duty intercoolers for air intake temperatures, and upgraded hoses to withstand increased heat. This step helps prevent overheating, especially when towing or climbing long grades.
Reinvent the Fuel System
If your original vehicle was petrol-powered, a complete fuel system conversion is in order. Even diesel vehicles may need upgrades to support the demands of a Cummins.
Key changes include installing a diesel-compatible fuel tank, replacing fuel lines, and fitting a high-capacity lift pump. If you’re using a P-pump or VP44, fuel delivery consistency is crucial. Add water separators and quality fuel filters to extend injector life and avoid clogging.
Tame the Tangle: Wiring and Electronics
Wiring can seem overwhelming, but it’s one of the most important parts of a successful Cummins swap. Whether you’re running a 12-valve with basic wiring needs or a 6.7L with complex electronics, integration is key.
Consider purchasing a standalone wiring harness tailored to your vehicle model. You’ll need to handle ECU communication, integrate the instrument cluster for accurate readings, and reroute sensor wires. Done correctly, it allows your new engine to talk to the rest of your vehicle with minimal hassle.
Breathing Room: Exhaust and Turbo Setup
Your Cummins will need room to breathe—and that means a serious exhaust upgrade. Start with a 4-inch turbo-back exhaust system that allows efficient airflow and keeps exhaust gas temperatures in check.
Be mindful of turbo positioning, downpipe routing, and whether your exhaust will clear crossmembers and driveshafts. A smoother path for airflow equals better performance and fuel efficiency.
Reinforce the Driveline
The torque output of a Cummins can easily overwhelm a weak driveline. Inspect your driveshafts, U-joints, and carrier bearings to ensure they can handle the added stress.
You may need to upgrade your axles or re-gear for improved performance. This is especially true if your build is aimed at towing or off-roading. Better to address potential weak points now than after they break on the trail.
Think Long-Term: Planning Your Conversion in Phases
One of the smartest things you can do before starting a Cummins conversion is to create a plan. What’s your end goal—reliability, performance, towing capacity? Build in phases, starting with essentials like the engine, transmission, and mounts. Follow with the cooling system, fuel delivery, and wiring, and round things out with driveline and suspension upgrades.
The right Cummins conversion parts will support each step of your build and help avoid setbacks or compatibility issues. And remember, quality components aren’t just a luxury—they’re an investment in your vehicle’s future performance.
Final Thoughts
A Cummins conversion can transform your truck from stock to unstoppable, but it’s not something you want to rush. Replacing the right components at the right time—starting with the engine, transmission, and structural parts—will give your build the strength and stability it needs.
From cooling and wiring to fuel delivery and mounts, every detail counts. Prioritise quality, plan strategically, and don’t underestimate the power of solid preparation. When all the parts come together, you’ll have a machine that’s not only powerful—but built to last.