What Actually Happens During a Diesel Engine Overhaul? A Step-By-Step Breakdown

When your heavy-duty diesel engine starts losing power, burning oil like it’s going out of style, or simply doesn’t sound right anymore, you’ll probably hear your technician recommend an engine overhaul. Sounds serious, right? It is. But it might just be the smartest financial move you can make.

Most truck owners or fleet operators aren’t mechanics, and that’s okay. But being left in the dark about what you’re paying for isn’t. That’s why today, we’re taking you behind the scenes with a step-by-step breakdown of a diesel engine overhaul, straight from the experts at Black Dog Truck and Trailer Repair in Leduc, Alberta.

Let’s pull that engine apart—figuratively, of course—and show you what’s happening.

What Is a Diesel Engine Overhaul?

Think of a diesel engine overhaul as a complete overhaul of your engine's entire system. It’s not just a tune-up. It’s not slapping in a new set of injectors or replacing a leaky gasket. It’s a detailed process where your engine is disassembled, inspected, cleaned, rebuilt, and tested to perform like new—or sometimes even better.

By restoring the heart of your truck, you avoid the massive expense of full engine replacement or buying a new truck altogether. Overhauls extend the lifespan of your vehicle, improve fuel efficiency, and restore lost power—all while keeping your operation moving.

The Full Diesel Engine Overhaul Process

1. Initial Engine Diagnostics

Before a wrench touches your engine, the shop starts with a comprehensive inspection. Sometimes, the symptoms you’re noticing might not require a complete overhaul. Technicians will typically perform:

• Compression tests to measure engine health
• Blow-by tests to check for worn piston rings or liners
• Oil sampling to detect contaminants or metal shavings
• ECM diagnostics to read active and inactive fault codes

At Black Dog Truck and Trailer Repair, they’ll sit down with you to explain their findings and give clear recommendations. If an overhaul makes sense, they’ll outline the scope and cost up front.

2. In-Frame vs. Out-of-Frame Overhauls

Here’s where the path splits.

In-Frame Overhaul

• Performed with the engine still in the chassis
• Less labour-intensive, lower cost
• Limited access to certain components

Out-of-Frame Overhaul

• The engine is entirely removed from the truck
• Allows complete inspection and rebuild of all engine components
• Higher cost but more thorough and longer-lasting

Your technician will help you weigh the pros and cons based on your engine’s condition, mileage, and operational goals.

3. Engine Removal (For Out-of-Frame Overhauls)

For out-of-frame jobs, your engine is carefully:

• Disconnected from wiring, hoses, fuel lines, and drivetrain
• Lifted out of the chassis using an engine hoist or crane
• Mounted on a stand for complete teardown

Removing the engine allows full access to internal and external components, ensuring nothing gets overlooked.

4. Complete Engine Disassembly

Now the real work begins. Technicians break your engine down to its bare block, removing:

• Cylinder head
• Pistons and rods
• Crankshaft
• Camshaft
• Bearings
• Seals and gaskets
• Oil pump and water pump
• Fuel system components

Everything is tagged, cleaned, and prepped for inspection.

5. Inspection & Measurement of Every Component

After the engine is fully disassembled, every part is measured and inspected for wear, damage, and structural integrity. This step ensures that nothing is reused unless it’s 100% up to spec. Inspection includes:

• Cylinder bore measurement
• Crankshaft and camshaft journal measurement
• Valve and seat inspection
• Piston and rod evaluation

If components can’t meet OEM specifications, they’re either replaced or machined.

6. Machining and Refurbishment

Reusable parts are sent off for precision machining, which may include:

• Cylinder reboring and honing
• Crankshaft grinding and polishing
• Valve seat cutting and head resurfacing

This ensures every component fits perfectly when reassembled, giving your engine factory-fresh tolerances.

7. Parts Replacement and Upgrades

Once machining is complete, worn components are replaced with high-quality OEM or premium aftermarket parts, including:

• New pistons and rings
• Cylinder liners
• Main and rod bearings
• Oil pump
• Gaskets and seals
• Water pump
• Injectors or injector cups (if required)

Some overhauls include turbocharger replacements, ECM recalibration, or upgraded fuel system components for enhanced performance.

8. Engine Reassembly

With parts ready and machined components back in spec, technicians carefully reassemble your engine. Every bolt is torqued to manufacturer specifications. Every clearance is double-checked. Every gasket and seal is professionally installed to ensure leak-free operation. Reassembly includes:

• Reinstalling the crankshaft, camshaft, and pistons
• Setting proper bearing clearances
• Rebuilding the cylinder head and valve train
• Refitting all external components

9. Testing and Validation

Before your engine ever sees road duty, it’s thoroughly tested, including:

• Compression tests
• Oil pressure validation
• Injector and fuel system checks
• ECM recalibration and tuning

At Black Dog Truck and Trailer Repair, they ensure that your engine runs properly.

10. Reinstallation and Final Checks (Out-of-Frame Only)

For engines pulled from the chassis, the rebuilt powerplant is carefully:

• Lowered back into the truck
• Reconnected to all wiring, hoses, and drivetrain
• Refilled with fresh fluids
• Tested again under load

The result? Your engine is ready to roll with restored power and reliability.

How Long Does the Overhaul Take?

On average, you should expect:

• 5 to 10 days for in-frame overhauls
• 10 to 20 days for out-of-frame overhauls

Delays may occur based on:

• Parts availability
• Engine complexity
• Shop scheduling

How Much Will It Cost?

Depending on the extent of the overhaul and parts used, expect to invest:

• $10,000 – $20,000 for basic in-frame jobs
• $20,000 – $45,000+ for complete out-of-frame rebuilds

While that might seem steep, replacing your truck is a fraction of the cost, especially if you’re running late-model Class 8 equipment.

Why It’s Worth Every Dollar

A professional diesel engine rebuild does more than extend engine life. It:

• Restores lost power and torque
• Improves fuel efficiency
• Reduces the risk of catastrophic failure
• Protects your uptime and profitability

A quality overhaul can give you hundreds of thousands of trouble-free miles, keeping you earning instead of sitting in a shop or shopping for a new truck.

Protecting Your Overhaul Investment

The best way to protect your investment is through proactive maintenance:

• Follow proper break-in procedures
• Stick to oil and coolant change intervals
• Schedule regular inspections
• Address small issues early

A well-maintained, overhauled engine can go the distance—again and again.

Final Thoughts

A diesel engine overhaul isn’t just “throwing parts at a problem.” It’s a comprehensive, precision-driven process that breathes new life into your engine.

Whether you run a single truck or manage a fleet, trust the Black Dog Truck and Trailer Repair team in Leduc, Alberta, to deliver honest advice, expert craftsmanship, and top-tier results.

Book Your Engine Overhaul Consultation Today

If you think your diesel engine is on its last legs, don’t wait until failure forces you to act. Contact Black Dog Truck and Trailer Repair today for a professional assessment and to find out if an engine overhaul is the right solution for your business.