Are trailer bearings all the same?

No. Trailer bearings aren’t all the same. They vary by spindle size, bearing type, seal style, and hub configuration, so you must match the bearing assembly to your trailer’s hub and axle.

Trailer bearings sit at the heart of wheel hubs and keep the wheels turning smoothly while handling loads, water intrusion, and debris. Most trailers use either traditional cup-and-cone tapered roller bearings or sealed bearing hubs. The differences you’ll encounter when replacing bearings include the spindle diameter, inner/outer race sizes, seal design, and whether a hub is serviceable or sealed for life. This article explains how to identify the correct bearings for your trailer and how to install them safely.

What makes trailer bearings different

Understanding these distinctions helps you avoid misfit parts that can lead to heat, wear, or failure on the road.

Bearing type and construction

Two main families cover most trailers: conventional cup-and-cone tapered roller bearings and sealed bearing hubs. Cup-and-cone hubs use separate inner and outer races with rollers and require regular lubrication and a precise adjustment. Sealed hubs use a pre-lubricated bearing unit and typically don’t require repacking, though they may have a limited service life.

Spindle diameter and inner/outer race sizes

Bearings come in different inner race bores (the hole that fits the spindle), outer race diameters, and overall width. The most common trailer spindle sizes are 1 inch and 1-1/4 inch, with some heavy-duty trailers using larger spindles. Always match the bearing kit to the exact spindle size and the hub’s bore measurements.

Seal design and cap configuration

Seal designs vary in diameter and style. Some hubs use simple grease seals that fit into the hub end; others use integrated bearing units with seals. The seal must fit the hub correctly to keep grease in and contaminants out.

Hub compatibility and configurations

Hubs differ in flange bolt patterns, the number of studs, and whether the hub is a drum or disc brake type. Ensure the part you buy matches your hub’s configuration, including the seal type and whether you have a serviceable or sealed-for-life hub.

Before replacing bearings, check the markings on the bearing or hub, measure the spindle bore, and confirm compatibility with your trailer’s hub. If you’re unsure, consult the trailer manufacturer or a parts specialist with the old bearing in hand.

How to identify your bearing size

The following steps help you determine the correct size and type prior to purchasing replacements.

1. Inspect the bearing numbers stamped on the races or on the hub. Note inner/outer diameters and any part numbers.


2. Measure with calipers: determine the inner race bore (spindle size), outer race diameter, and the width of the bearing or bearing seal.


3. Determine the hub type: decide whether you have a conventional cup-and-cone hub or a sealed bearing hub. If your hub is sealed, you’re typically selecting a complete bearing unit rather than separate races.


4. Check the axle spindle diameter: confirm whether your spindle is 1" or 1-1/4" (most trailers use these sizes).


5. Cross-check the measurements and part numbers with a reputable parts retailer or your trailer manual to confirm compatibility.

If you get the fit right, you’ll gain longer bearing life, better heat management, and safer towing.

Practical replacement steps

After identifying the correct bearings, follow these general steps to perform a safe replacement. This overview covers the main tasks; consult your trailer’s service manual for torque specs and any model-specific steps.

1. Secure the trailer, chock wheels, and drain old grease if applicable.


2. Remove the hub and take out the old bearings, seals, and races. Inspect the spindle for scoring or wear.


3. Clean the hub and inspect for corrosion. Replace any damaged components and consider replacing the grease seal if worn.


4. Install the new inner race, then the outer race if using a serviceable hub. Repack cups with grease if applicable (cup-and-cone hubs).


5. Reinstall seals, hub cap, and torque the spindle nut to the specified value. Reinstall cotter pin or locknut as required.

With proper bearings and assembly, you reduce risk of heat buildup and early wear, and you protect your tires and trailer load.

Summary

Trailer bearings are not universal. They differ in spindle size, bearing type, seal design, and hub configuration. To ensure a safe, reliable replacement, identify the exact size and type from your hub markings or manual, and use a compatible bearing kit. When in doubt, seek assistance from a trailer parts retailer or a professional mechanic to verify compatibility and installation procedures.

How do I know what bearings to get for my trailer?

To find the right trailer bearings, the easiest method is to check the part number stamped on the old bearings after removing the hub. If the numbers are unreadable, you will need to take precise measurements of the inner and outer bearings and the spindle using a digital caliper.
 
This video demonstrates how to remove the hub to access the bearings: 55sTrailerPartsYouTube · May 9, 2024
Method 1: Use the part number 

1. Safely jack up the trailer: on the frame, not the axle, so the wheel is off the ground. 
2. Remove the wheel and tire: and the grease cap. 
3. Remove the cotter pin, nut, and washer: securing the spindle. 
4. Remove the hub: to expose the inner and outer bearings. 
5. Find the part number: stamped on the metal of both the inner and outer bearings and the corresponding races. 
6. Use these numbers: to purchase the correct replacements. 

This video explains how to find the part number on the bearings: 1mTowing & TrailersYouTube · Jun 28, 2019
Method 2: Measure the bearings 

1. If no numbers are visible, you must take measurements to find replacements. 
2. Use a digital caliper for the most accurate results, as a tape measure is not precise enough. 
3. Measure the inner diameter and outer diameter of both the inner and outer bearings. 
4. Also, measure the width of each bearing. 
5. Note the spindle's measurements where the bearings sit, as this is often necessary for matching. 
6. Compare your measurements to a bearing size chart or provide them to a trailer parts supplier. 

This video shows how to use a digital caliper to measure the inner diameter of a bearing: 57sBlueswift AxlesYouTube · Mar 25, 2025
Additional tips

• When ordering, always get the numbers for both the inner and outer bearings, as they are typically different sizes. 
• Don't forget the corresponding seal, which is also a specific size and is needed to complete the set. 
• If you have a homemade trailer, it's crucial to get the measurements right, as the axle may not be standard. 
• If you are unsure or uncomfortable with any of these steps, consult a professional mechanic or trailer parts specialist. 

Can I spray WD-40 on a wheel bearing?

No, you should not use standard WD-40 on wheel bearings as a lubricant because it will strip the protective grease, causing premature wear and damage. WD-40 is a water displacer and degreaser, not a long-lasting lubricant. For lubrication, use a proper wheel bearing grease, or a WD-40 Specialist product like white lithium grease or gel lube if you need a spray lubricant. 
Why standard WD-40 is bad for wheel bearings

• It's a degreaser: Its primary function is to cut through and dissolve grease, which is what protects wheel bearings. 
• Low viscosity: It's too thin to stay in the bearing and provides no lasting lubrication, leading to metal-on-metal contact and wear. 
• Attracts dirt: It can leave a residue that attracts dirt and grime, further increasing wear. 

What to use instead

• Wheel bearing grease: This is the best choice for long-term lubrication and protection. You can purchase it at any auto parts or tractor supply store. 
• WD-40 Specialist products: If you prefer a spray, use a product specifically designed for lubrication from the WD-40 line: 
  • Specialist White Lithium Grease: This is a heavier-duty, water-resistant lubricant for high-pressure applications. 
  • Specialist Gel Lube: This is a non-drip gel that resists water, moisture, and rust for extended periods. 

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What is the difference between 6903 and 6803 bearings?

The main difference is their dimensions. A 6803 bearing is 17x26x5mm, while a 6903 bearing is 17x30x7mm. The 6903 is wider and has a larger outer diameter, designed for applications requiring higher load capacity. They are not interchangeable.

Are all trailer wheel bearings the same?

The hub holds 2 bearings, the inner bearing (inside the hub flange, closest to the trailer center), and the outer bearing. Since one size trailer wheel bearing does not fit all, to determine the correct size wheel bearing for your trailer, you'll have to take apart the hub.