A 98 K1500 owner explains why he chose an LLY Duramax/Allison swap instead of the more common 12-valve or LS route, and walks through the real-world process of getting the drivetrain into an OBS cab without a body lift. He shares sourcing the truck, pulling the engine/transmission, building custom mounts, and how accessible the swap is (turbo/downpipe included). The build is driven by reliability goals, with future plans around 450–500 hp, plus worries about wiring, LOY head gasket risk, and inspection/registration. He’s also documenting everything on YouTube (DK Custom Trucks).
Today’s guest walks us through fitting an LLY Duramax in his 1998
Sierra! We ask him what got him started down the diesel road, challenges
with fitting it into the truck, and his ultimate plans for power and
performance!
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“Duramax” refers to GM’s diesel engine family, commonly swapped into trucks like the GMC Sierra. It’s popular because the aftermarket supports it heavily with fueling, turbo, and cooling upgrades.
"...everybody does like their LS swaps to come and swap..."
An LS swap is when people put a GM LS V8 engine into another truck. It’s popular because it’s well-supported with parts and tuning, and the speaker is saying most people go that route instead of a diesel swap.
An “LS swap” is when enthusiasts replace an older engine with a GM LS-series V8 (often for reliability, aftermarket support, and modern drivability). The speaker contrasts LS swaps with Duramax swaps, implying the Duramax route is less common for this platform.
"They direct me right to an LS swap or something like that. ... Like how did you like with doing a Duramax swap? ... I was actually out here working on the truck."
A “’98 Sierra” is a 1998 GMC pickup truck. People often choose older Sierras because they’re easier to modify for swaps.
A “’98 Sierra” is a 1998 GMC Sierra pickup, which is a popular platform for engine swaps. Older GM trucks like this are often used because the engine bay and wiring can be adapted to new powertrains.
"it needed like some exhausts worked on it had true tools on it headers. So I was getting exhaust manifolds tightened back up because they were leaking the exhaust"
Exhaust manifolds are the parts that collect exhaust from the engine and send it down the exhaust system. If they’re leaking, you’ll often hear it and may smell exhaust.
Exhaust manifolds route exhaust gases from the engine cylinders to the rest of the exhaust system. If they leak, you can get noise, reduced performance, and exhaust fumes in the engine bay.
"it needed like some exhausts worked on it had true tools on it headers. So I was getting exhaust manifolds tightened back up"
Headers are upgraded exhaust parts that help gases flow out more easily. They can be louder and, if not sealed right, they can leak exhaust.
Headers are aftermarket exhaust components that replace the factory exhaust manifolds to improve exhaust flow. They’re commonly used on swaps and performance builds, but they can require careful fitment and sealing to prevent leaks.
"So I started working on all that and then I noticed the core support was getting rotted. So I was like, man, I want to just pull that off and then order a new one."
The core support is the front metal frame that holds the radiator and cooling parts. If it’s rusted out, the radiator can be loose or misaligned, and the truck needs repair.
The core support is the structural panel at the front of the vehicle that holds the radiator and related cooling components. When it’s “rotted,” it can affect cooling alignment and structural integrity, so replacing it is a common repair on older trucks.
"Have you been around people that have had a Duramax? Like, have you felt the power of the torque, all that sort of stuff?"
Torque is the twisting force an engine produces, and it’s especially important for diesels because they tend to make strong torque at lower RPM. That’s why people associate diesel engines with effortless towing and strong acceleration under load. The speaker is connecting torque to the appeal of a Duramax swap.
"the sky is really the limit on diesel engines with what you can do with like air and fuel and torque and power and all that sort of stuff."
“Air and fuel” is shorthand for the combustion inputs that determine how much power a diesel can make. More air (often via turbo boost) and the right amount of fuel can increase torque and horsepower, but too much fuel without enough air can create smoke and heat. The speaker is describing the core tuning concept behind diesel performance.
"the sky is really the limit on diesel engines with what you can do with like air and fuel and torque and power and all that sort of stuff."
Diesel engines are a type of engine that makes power by compressing air and using fuel to ignite it. People like them because they often pull hard and can be tuned for more power. The tradeoff is that you have to do it safely.
Diesel engines run on compression ignition and typically produce high torque compared with many gasoline engines. In the aftermarket, “diesel engines” are often discussed in terms of tuning for air/fuel delivery, boost control, and efficiency. The speaker frames diesel potential as being limited mainly by how much air and fuel you can safely manage.
"So I've always been the GM guy. So I've always liked the diesel engines that come with the GM, not necessarily the 65, but the Duramax for sure."
GM is General Motors, the company that makes the Duramax diesel engines. When someone says they’re a “GM guy,” they usually mean they like GM products and the way they’re built and supported. In this case, it points to Duramax.
GM refers to General Motors, the automaker behind the Duramax diesel family. The speaker says they’ve always been a “GM guy,” meaning they prefer GM’s diesel platform and the way it’s supported by parts and tuning communities. This helps listeners understand why Duramax is a natural target for a swap.
"...he had just bought a brand, almost brand new truck. Gasser instead of, you know, getting something older."
“Gasser” just means the truck runs on gasoline. Diesel trucks are different, and the whole point here is moving toward a diesel setup.
“Gasser” is slang for a gasoline-powered vehicle, as opposed to a diesel. In the context of this episode, it highlights that the coworker was considering staying with gasoline rather than buying an older diesel truck.
"Like I, you know, popped the hood, unscrewed the cap, flipped it over while the truck was idling."
“Popped the hood” is a common phrase meaning the person opened the engine bay to inspect components. It often precedes checks like fluid levels, leaks, or whether the engine is running normally.
"Like I, you know, popped the hood, unscrewed the cap, flipped it over while the truck was idling. It didn't budge, spin, twist, nothing."
Idling means the engine is running but the truck isn’t moving. It’s a good time to check how the engine behaves because you’re not dealing with driving forces.
Idling means the engine is running while the truck is stationary. Checking behavior “while idling” is useful because it isolates engine operation from driving loads, helping diagnose issues like vibration, misfires, or abnormal movement.
"And they actually make a motor mount kit, but it's like $219 and I was like, well, I have the steel..."
A motor mount kit is an aftermarket (or sometimes OEM-style) set of brackets and hardware designed to adapt an engine to a specific chassis. The speaker mentions one exists for this swap, but they choose to fabricate their own mounts instead to save cost and use their existing steel and welding skills.
"Um, I could take the turbo out through the, through the engine bay, I wouldn't have to pull the cab or anything like that."
A turbo is a device that helps the engine make more power by pushing extra air into it. Here, they’re saying it’s easier to remove because there’s room in the engine bay.
A turbocharger (often shortened to “turbo”) forces more air into the engine, improving power and efficiency—especially on diesel engines. The transcript mentions removing the turbo through the engine bay, which is a serviceability and packaging point.
"Everything is very accessible. I could probably change the downpipe on it right in the end, right in the truck as well."
A downpipe is part of the exhaust right after the turbo. It can be hard to install because it has to line up correctly and fit in tight spaces.
A downpipe is the exhaust pipe section that carries exhaust gases from the turbo to the rest of the exhaust system (often to the catalytic converter and muffler). It can be tricky to fit due to heat shielding, clearance to the frame, and alignment with the turbo outlet.
"Maybe I want to build a transmission, but kind of wait till down the road to make those decisions. ... I know when you start pushing into the like 500 inch range, you're going to start looking at, uh, probably building a transmission..."
The transmission is what sends power from the engine to the wheels. If the engine makes much more torque than before, the transmission may need upgrades to survive.
A transmission is the gearbox that transfers engine power to the driveshaft and wheels. When torque rises (like with a diesel swap), the transmission often becomes the limiting component, so the speaker is considering whether to build/upgrade it.
"...350 gas engine that made... 250 horsepower and 300 foot pounds of torque to something that makes 300 and some horsepower and almost 600 foot pounds of torque."
Horsepower is a measure of how much power the engine makes. The speaker is comparing horsepower to explain why the truck’s drivetrain might need to handle more stress.
Horsepower is a measure of engine power output, often associated with how fast a vehicle can accelerate at higher speeds. The speaker compares horsepower before and after the diesel swap to show the performance jump and why supporting components may need upgrades.
Term
1000
"...especially the, uh, 1000 that's in this, the, the five speed."
“1000” here likely means a big-number durability target—like how much twisting force the drivetrain can handle. They’re saying the truck’s current setup (the five-speed) may be right at that kind of limit.
In this context, “1000” most likely refers to a torque rating or a component spec (often used in diesel circles to mean “1000 lb-ft capable”). The speaker is tying that threshold to the strength of the five-speed transmission.
"I, I'm a little worried about that because I don't want to, I don't want to grenade it immediately, but I think that like probably around 400, 450 foot pounds..."
“Grenade it” is slang for catastrophic failure—something breaks suddenly and severely. Here, the speaker is worried the transmission could fail immediately if they push power too far without upgrading.
"[918.9s] so much better to do it when you want to do it versus when you're
[922.3s] forced to because a clutch burned up or something like that.
[925.1s] And now you got to spend the money to do it."
“Clutch burned up” means the friction surfaces inside the transmission got too hot and failed. It usually happens when the transmission is working harder than it was designed for.
“Clutch burned up” describes clutch friction material overheating and failing, which is common when a transmission is pushed beyond its limits. It’s often used as a cautionary example: if you don’t upgrade the drivetrain proactively, you can end up paying for repairs later.
"He was just going to fix the truck. And then that was before he broke the bolt on the caliper that really did it in and I got really discouraged for a while."
A “caliper” is the brake component that clamps brake pads against the rotor. The speaker says a bolt on the caliper was broken, which derailed the plan and caused discouragement—illustrating how small hardware issues can stall a project.
"...all the stuff in the headliner that's, you know, for your dome lights, all that stuff should be in there as well."
The “headliner” is the interior ceiling panel. The speaker notes that wiring located in the headliner—specifically for dome lights—will be transferred, which helps ensure the interior lighting works correctly after the swap.
"Like it's a 60 40 bench. So like the 40% was the driver seat and the 60% was the passenger seat."
A 60/40 bench seat is a wide bench seat split into two parts. One side is bigger than the other, and it can make it easier to move around or access the middle.
A “60/40 bench” is a split bench seat where one side is 60% of the width and the other is 40%, typically allowing access to the center area or easier entry/exit. The speaker uses it to describe how the truck’s seating was configured while other seats were missing.
"They have tons of choices for rods, cranks, pistons, valve train upgrades, tons of different things."
Pistons are the components that move inside the cylinders and transfer force to the connecting rods. In diesel performance builds, piston upgrades can improve durability under higher boost/fueling and help manage heat and pressure.
"There's a couple odds and ends guys that have done it, but nobody's
showing you how how they've done it.
It's just like, Hey, here it is."
An engine swap is when you put a different engine into a vehicle than it originally came with. It usually takes a lot of custom work, which is why it can be hard to find clear instructions.
An engine swap is when someone removes the original engine and installs a different one, often requiring custom wiring, fabrication, and compatibility work. This segment is discussing why Duramax swaps into OBS trucks are hard to document and replicate.
"...if there's any, um, what I was doing, you know, how I built the motor mounts, you know, the custom, uh, CAD, CAD, cardboard aiding development."
Motor mounts are the parts that bolt the engine to the truck. When you swap an engine, you usually have to make new mounts so the engine sits in the right spot.
Motor mounts are the brackets and isolators that hold an engine to the truck’s frame and control engine movement. In an engine swap, mounts often need to be custom-built or modified to match the new engine’s mounting points and to keep driveline alignment correct.
"off of the, um, the Duramax on that truck and, uh, I want to supercharge that someday."
Supercharging means using a device to push more air into the engine. More air usually means more power, but it can also require supporting upgrades so the engine stays healthy.
To “supercharge” means adding a forced-induction system (a supercharger) that compresses air before it enters the engine. On diesel builds, it’s often used to increase airflow and power, but it also raises stress on fuel delivery, cooling, and engine components.
"And so I was like talking with him about taking, you know, like an older engine and putting it into a newer truck."
This is when someone replaces the engine in a newer truck with an older engine. The goal is usually to combine a “proven” engine with a newer truck that’s nicer to live with.
This is an engine swap concept: moving a proven older powerplant into a newer chassis/body. It’s popular in diesel circles because older engines can have simpler designs, strong aftermarket support, and known reliability, while newer trucks can offer better comfort, safety, and emissions equipment.
"It's now with the aftermarket and this kind of being your first diesel
truck that you're, that you're going to be like going really heavy into like
what have you thought about the aftermarket side like with companies"
“Aftermarket” just means upgrades made by companies other than the truck maker. For diesel trucks, that can include tuning and performance parts.
“Aftermarket” means parts and software made by companies other than the original manufacturer. For diesel trucks, the aftermarket is huge—covering tuning, fuel system upgrades, exhaust, and supporting hardware to safely handle more power.
"So that stuff looks really cool, but I was also following the, uh, WC fab stuff [2364.7s] because of the, uh, square body stuff. Cause I'm hoping for one of my next projects to be a Duramax swap square body, though those are becoming a dime a dozen as well."
“Square body” is a nickname for certain 1970s–1980s GM truck designs with more boxy styling. In the Duramax-swap world, square bodies are popular because they look great with modern diesel power and there’s an established community for fitment and fabrication.
"I was like, I hope this truck starts cause you know, I had like, it was trying to hydrolock itself the entire time I'm trying to start it…"
Hydrolock is when water gets into the engine’s cylinders. Since water can’t compress like air, it can cause serious engine damage.
Hydrolock happens when liquid (like water) gets into a cylinder and can’t be compressed. That can bend internal engine parts or prevent the engine from turning over safely.
"…single cab long box four wheel drive on 33s. It was a pretty sweet looking truck."
“33s” means the tires are about 33 inches tall. Bigger tires can help off-road, but they can also change how the truck drives and uses fuel.
“33s” is shorthand for 33-inch tires, a common way to describe tire size in truck culture. Larger tires can improve ground clearance and off-road capability but may affect ride quality, gearing, and fuel economy.
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