When your diesel engine needs a replacement part, one of the first decisions you’ll face is whether to choose an OEM or aftermarket option. Both are available for many heavy-duty diesel engine applications, but how do you know which is right for your repair?
While price is often a major consideration, it’s not the only factor that matters. The right replacement part should fit your engine, meet your repair needs, and provide the quality and reliability you’re looking for.
Before making a purchase, here are five important factors to consider: fitment, quality, price, warranty, and availability.
What Is the Difference Between OEM and Aftermarket Diesel Parts?
Understanding the difference between OEM vs. Aftermarket diesel parts can help you make a more informed purchasing decision.
OEM (Original Equipment Manufacturer) Parts
OEM replacement parts are supplied under the original engine manufacturer’s brand, such as Cummins, Caterpillar, or Detroit Diesel. These parts are intended to meet the manufacturer’s specifications for their designated engine applications.
Aftermarket Diesel Parts
Aftermarket parts are replacement components offered by manufacturers or suppliers outside the original engine manufacturer’s brand. These parts are designed for specific engine applications and may provide an alternative to OEM-branded components.
Aftermarket options are available for a wide range of diesel engine components, including cylinder heads, fuel injectors, turbochargers, pistons, gaskets, and engine rebuild kits.
It’s also important to understand that OEM vs. aftermarket is not the same as new vs. remanufactured. Both OEM and aftermarket product lines can include new and remanufactured components.
Neither option is automatically the best choice for every repair. The quality, specifications, and value of a replacement part depend on the specific product and manufacturer.
1. Fitment: Will the Part Work With Your Engine?
Before comparing prices or manufacturers, make sure you’re looking at the correct part for your engine.
Two trucks with the same engine model may require different components depending on the engine’s configuration, production date, or application.
For example, if you’re shopping for a replacement cylinder head for a Cummins ISX15, knowing the engine model alone may not be enough to identify the correct replacement. Different engine configurations can require different cylinder head part numbers.
To help confirm compatibility, you may need:
- Engine Serial Number (ESN): Helps identify your specific engine and its configuration.
- CPL or Arrangement Number: Depending on the engine manufacturer, these identifiers can help narrow down the correct components.
- Original Part Number: Provides a starting point for identifying compatible OEM and aftermarket replacements.
- Part-Number Cross-References: Can help identify alternative replacement options, but should be verified against the engine application.
Don’t assume that a part will fit simply because it’s listed for your engine model. Confirming compatibility before ordering can help prevent incorrect purchases, installation delays, and unnecessary downtime.
The takeaway: Start with the correct engine application and part number before comparing OEM and aftermarket options.
2. Quality: Look Beyond the OEM or Aftermarket Label
Not all replacement diesel engine parts are manufactured to the same standards. That’s why it’s important to evaluate the specific manufacturer and product rather than relying solely on whether it’s labeled OEM or aftermarket.
When comparing replacement parts, consider:
- Manufacturer Reputation: Does the manufacturer have an established history of producing diesel engine components?
- Product Specifications: Are the materials, dimensions, and application details available?
- Quality Control and Testing: Does the manufacturer provide information about its inspection or testing processes?
- Performance Claims: If a product is advertised as meeting or matching OEM specifications, what information supports that claim?
For example, when comparing OEM and aftermarket fuel injectors, you may want to review available specifications, testing information, and manufacturer documentation before deciding which option meets your needs.
Some aftermarket manufacturers offer detailed product information and documented testing, while others provide limited technical details. The same level of information may not be available for every product.
If you’re unsure about a particular component, ask your parts supplier for additional information before purchasing.
The takeaway: Look beyond the label. Understanding who manufactures the part and what supports its quality claims can help you make a more confident decision.
3. Price: Compare More Than the Price Tag
Price is often one of the biggest factors when choosing between OEM and aftermarket diesel parts, especially when you’re managing repair costs and trying to get your truck back on the road.
Aftermarket parts may be available at a lower purchase price than OEM-branded alternatives, but that doesn’t necessarily mean every aftermarket option will cost less.
It’s important to compare products with similar specifications, condition, and included components.
For example, consider two engine rebuild kits designed for the same application.
One kit may have a lower advertised price but include fewer components. Another may cost more upfront but include additional gaskets, seals, bearings, or other parts needed to complete the repair.
If you need to purchase those missing components separately, the lower-priced kit may not provide the savings you initially expected.
When comparing prices, consider:
- The cost of the replacement part or kit
- Components and hardware included
- Additional parts required for installation
- Shipping costs
- Applicable core charges for certain components
- Whether you’re comparing new or remanufactured products
Core charges are especially important when shopping for certain components, such as fuel injectors and turbochargers. Depending on the supplier’s policy, you may pay an additional refundable core charge until an eligible used component is returned and accepted.
Comparing the complete purchase cost can give you a more accurate picture of what each option will cost.
The takeaway: Don’t compare price tags alone. Make sure you’re comparing equivalent products and accounting for everything needed to complete the repair.
4. Warranty: Know What’s Covered Before You Buy
A warranty can provide additional peace of mind when purchasing replacement diesel engine parts, but coverage can vary significantly depending on the manufacturer, supplier, and specific product.
Don’t assume that all OEM parts have the same warranty or that all aftermarket parts offer identical coverage.
Before purchasing, review the written warranty terms and ask:
- Coverage Period: How long is the warranty valid?
- Parts and Labor: Does the warranty cover only the replacement part, or is labor included under certain conditions?
- Installation Requirements: Are there specific installation procedures or qualifications required to maintain coverage?
- Exclusions: What types of damage, operating conditions, or failures are excluded?
- Claims Process: What documentation is required to submit a warranty claim?
For example, if you’re replacing a turbocharger, it’s worth understanding whether the warranty covers the component itself, whether any labor reimbursement is available, and what installation or maintenance requirements apply.
Keeping your purchase records, installation documentation, and other required information can also help if you need to file a claim later.
The takeaway: Review the warranty before buying so you understand what is covered and what steps are required if a problem occurs.
5. Availability: How Quickly Can You Get the Part?
When your truck is out of service, getting the correct replacement part quickly can be just as important as finding the right price.
A part may be listed on a supplier’s website without necessarily being in stock and ready to ship.
Before placing an order, consider:
- Stock Status: Is the part physically in stock or does it need to be sourced?
- Processing Time: How long will it take before the order ships?
- Delivery Estimate: When is the part expected to arrive at your location?
- Complete Repair Availability: Are all additional components needed for the repair available?
For example, if you need a replacement cylinder head, receiving the head quickly won’t necessarily help if the required gasket set or head bolts won’t arrive until several days later.
Confirming availability for all required components can help you plan your repair more effectively.
It’s also worth comparing the expected delivery dates of OEM and aftermarket options. Depending on the part and supplier, one may be available sooner than the other.
The takeaway: Consider availability and expected delivery alongside price, especially when downtime affects your ability to get back on the road.
Before You Buy: A Quick Diesel Parts Checklist
Whether you’re purchasing OEM or aftermarket diesel engine components, take a moment to review these five questions:
- Fitment: Have I confirmed my engine application and the correct replacement part number?
- Quality: Do I know who manufactured the part and what product information is available?
- Price: Have I compared the included components, condition, and total purchase cost?
- Warranty: Do I understand the warranty coverage, requirements, and claims process?
- Availability: Have I confirmed stock status and expected delivery for everything needed to complete the repair?
Taking the time to answer these questions can help you avoid unexpected costs and make a more informed purchasing decision.
Finding the Right Diesel Engine Parts for Your Repair
Choosing between OEM vs. aftermarket diesel parts isn’t always a straightforward decision, as each option offers different considerations depending on your engine, repair needs, and budget.
At Highway and Heavy Parts (HHP), we understand how important it is to find the correct replacement components for your heavy-duty diesel engine. Whether you’re comparing aftermarket options, checking part compatibility, or looking for available components, our team is here to help you navigate your options.
Have questions about OEM or aftermarket options for your diesel engine? Call HHP at 844-304-7688 with your engine information and part number.
