Understanding 24377 and 6805 (25377) Bearings in Bottom Brackets

When it comes to bottom brackets designed for 24mm spindle cranks, two bearing sizes often come up: 24377 bearings and 6805 (25x37x7mm) bearings. Although 24377 and 6805 share similarities, their use in different bottom bracket systems affects performance, compatibility, and installation.

24377 bearing

6805-7mm with 24mm dust cover

24377 Bearings

The 24377 bearing (24x37x7mm) is often found in certain bottom brackets, such as Enduro’s PF30 conversion models. The crank spindles sit directly on the bearing inner rings without additional dust covers.

• Pros: Simple setup, no extra dust cover required, designed specifically for 24mm spindles.

• Cons: Reduced protection against water and dust, making the bearing more exposed and creaking, potentially shortening its lifespan.
  

6805 (25x37x7mm) Bearings

In contrast, Shimano’s classic approach is to use the 6805 bearing (25x37x7mm) with a 24mm dust cover sleeve. It applies to their entire Hollowtech bottom bracket range. The dust cover effectively reduces the inner diameter from 25mm to 24mm, allowing the crank spindle to fit while also adding a protective layer.

• Pros: Better sealing and protection against contamination, improved integration with 24mm spindles, it is serviceable and replaceable to help extend the bearing's lifespan.

• Cons: Slightly more complex setup due to the dust cover, the dust cover is made of Delrin plastic, which can wear or tear over time.

Tip: The 6805 bearing comes in two different widths: 6805-6 (25x37x6mm) and 6805-7 (25x37x7mm). Most bottom brackets use the 7mm version, while the 6mm version is commonly found in Campagnolo UT bottom brackets.

24377 and 6805 Replacement Options

If your current bottom bracket uses 24377 bearings, you have three main choices:

1. Direct Replacement – Replace with 24377 bearing range to keep the same setup.

2. Upgrade with 6805 + Dust Cover – Use our 6805 (25x37x7mm) bearings with 24mm dust covers, essentially creating the same setup as a Shimano-style BB90 bottom bracket (6805 bearings + 24mm dust cover)

3. Replace the Whole Bottom Bracket – This is often the best option. Installing a complete bottom bracket is always easier than pressing in individual bearings into the bottom bracket cups, and it also ensures that the cups and bearings are perfectly matched.

   
   

Important Note on Fitment

Bottom bracket manufacturers often machine their cups with specific tolerances to best fit their own bearings. This means that even if the dimensions match, your new bearings may not always fit snugly into existing cups; sometimes, even within the same brand, different batches can vary slightly. (refer to our blog article: Tolerance Mismatch Solution: Fix Bottom Brackets & Wheel Bearings Issues) For the smoothest installation and best durability, replacing the entire bottom bracket is usually the most reliable solution.