BSA 57-4306 Gearbox Sprocket B44 15T

( Brand: Bsa ), ( Manufacturer Part Number: 57-4306 ), ( Part Type: Sprocket ), ( Country Of Origin: United Kingdom )

Review BSA B44 Gearbox 15t Sprocket fits B25 B50 C25

The **BSA 57-4306 B25/B44/B50/C25 Gearbox with 15T Sprocket** is a meticulously engineered transmission unit designed for high-performance motorcycles, particularly those built on the legendary BSA Gold Star or similar British twin-cylinder platforms. This gearbox represents a refined blend of durability, precision, and adaptability, making it a sought-after component for both restoration projects and custom builds. Crafted from high-grade steel and featuring a robust cast-iron housing, the gearbox is built to withstand the demands of aggressive riding, ensuring smooth power delivery and longevity under heavy loads. The internal gearing is finely machined, with hardened steel cogs and optimized tooth profiles to minimize wear and maximize efficiency, whether paired with the stock BSA engine or a modified high-revving setup. The **15-tooth sprocket** integrated into the primary drive system is a critical element, offering a balanced powerband that complements both cruising comfort and spirited acceleration, depending on the final drive ratio selected. This gearbox is particularly well-suited for applications requiring a versatile transmission, as it accommodates a range of primary chain configurations (B25, B44, B50, or C25), allowing riders to fine-tune the gearing for their specific riding style, terrain, or engine modifications. Whether installed in a classic BSA Gold Star, a custom twin-cylinder bike, or a modern retro-inspired project, this gearbox delivers a seamless connection between the engine and the rear wheel, ensuring reliable performance and a satisfying driving experience. Its compatibility with aftermarket components and ease of maintenance further solidify its reputation as a premium choice for enthusiasts seeking both authenticity and performance.

### **Pros and Cons of Buying a BSA 57-4306 B25/B44/B50/C25 Gearbox with a 15T Sprocket**

1. **Compatibility with Classic BSA Motorcycles** The BSA 57-4306 gearbox is designed specifically for BSA models (such as the BSA Gold Star, A7, or other classic BSA bikes), ensuring a seamless fit without major modifications.

2. **Proven Reliability** BSA gearboxes from this era (1950s 1960s) were built to last, with robust construction and durable components. Many still function well in restored or modified bikes.

3. **Aftermarket Support** Due to BSA s historical popularity, replacement parts, rebuild kits, and restoration guides are widely available. This makes maintenance and repairs more accessible.

4. **Adjustable Gear Ratios** The inclusion of a 15T sprocket (often paired with a 36T or 38T chainring) allows for tuning the bike s performance. A smaller sprocket increases top-end speed and acceleration, which can be beneficial for modern riding styles.

5. **Classic Aesthetic & Vintage Appeal** For enthusiasts restoring or modifying classic bikes, using an original or period-correct gearbox enhances authenticity and resale value.

6. **Potential for Customization** The gearbox can be paired with modern components (such as a performance chain, upgraded bearings, or a custom clutch) to improve durability and performance.

7. **Lower Cost Compared to New Gearboxes** Purchasing a used or rebuilt BSA gearbox is often significantly cheaper than buying a new one, making it a budget-friendly option for restorers.

---

1. **Age-Related Wear** If the gearbox is used, it may have internal wear (such as worn synchronizers, stripped threads, or damaged bearings). A thorough inspection or rebuild may be necessary before installation.

2. **Limited Modern Compatibility** While BSA gearboxes work well with classic BSA engines, pairing them with modern engines (e.g., Japanese or custom-built) may require additional modifications, such as a different primary drive or clutch setup.

3. **Maintenance Requirements** Older gearboxes may require more frequent servicing, including gear oil changes, synchronizer adjustments, and bearing checks, compared to modern units.

4. **Potential for Misalignment Issues** If not properly aligned with the engine or transmission housing, the gearbox can cause excessive wear or even damage to the primary chain or clutch.

5. **Availability of Parts** While BSA parts are widely available, some rare or obsolete components (e.g., specific seals, synchronizers, or bolts) may be difficult to source, especially for very old models.

6. **Performance Limitations** The gear ratios in a stock BSA gearbox may not suit modern riding preferences (e.g., high-speed cruising or aggressive acceleration). A 15T sprocket helps, but further modifications (such as a different chainring or gear ratios) may still be needed.

7. **Risk of Counterfeit or Poor-Quality Units** Buying from untrusted sellers may result in a gearbox that has been rebuilt poorly or is a replica with substandard materials.

---

The BSA 57-4306 gearbox with a 15T sprocket is an excellent choice for **BSA enthusiasts, restorers, or riders seeking a classic, reliable transmission** for their vintage bike. Its compatibility, durability, and aftermarket support make it a strong option, especially if you plan to pair it with an original or restored BSA engine. However, potential buyers should **inspect the gearbox thoroughly for wear, ensure proper alignment with the engine, and consider rebuilding it if it s used** to avoid future issues.

For **modern or heavily modified bikes**, additional adjustments (such as a different chainring or gear ratios) may be necessary to optimize performance. If you prioritize **ease of maintenance and classic authenticity**, this gearbox is a solid investment. If you re looking for **cutting-edge performance or minimal maintenance**, a modern gearbox (e.g., from a Japanese bike or a performance aftermarket supplier) might be a better fit.

- **Buy if:** You own a BSA motorcycle, value classic components, and are willing to invest time in inspection/rebuild. A 15T sprocket is a good upgrade for better top-end performance.

- **Avoid if:** You need a gearbox for a non-BSA bike without modifications, prefer modern reliability, or want minimal maintenance.

- **Best Practice:** Purchase from a reputable seller (preferably with a rebuild history), inspect for wear, and consider professional servicing before installation. Pair it with a compatible clutch and primary drive for optimal performance.

Details:

Nice Used Part # 57-4306, Sprocket teeth: 14, Chain size: 520, Pitch: 5/8 X 1/4, Fits: BSA B25, B44 ,B50 C25 from 1965-73, Triumph TR25, T25 1968-71 57-4306 14 520 1/4 1965-73.

part #: 574306 price

• $39.95

specifications cyclevintagee:

• for models: B25, B50, C25
• brand: Bsa
• mpn: 57-4306
• part type: Sprocket
• country of origin: United Kingdom

seller cyclevintagee:

• city: Avondale, Pennsylvania
• # reviews: 40
• rating: 100.0%

general cyclevintagee:

• Parts Accessories > Motorcycle Scooter Parts Accessories > Other Motorcycle Scooter Parts Accessories
• condition: Used

payments cyclevintagee:

• wallet: Paypal
• money order
• credit card: Diners-club
• other

shipping cyclevintagee:

• type: Calculated
• transit: 4-9 Days
• service: USPS Ground Advantage
• class: Standard
• cost: $5.62
• carrier: USPS

returns cyclevintagee:

• time: 30 Days
• policy: Returns Accepted
• paid by: Buyer

offer cyclevintagee:

• availability: In Stock
• quantity: 1
• sold: 0
• started: March 8, 2026
• options: Ship-to-home

Bsa 57-4306 Is Similar To:

• 68-3093 Bsa A50 A65 Gearbox 18t Sprocket (63.6% similar) This is a brand new gearbox transmission sprocket...(August 1st, 2016)
• 65852 Unknown Bsa Triumph Gearbox W Lever Swingarm Used Vintage Ahrma (30.8% similar) ...(October 6th, 2016)
• Bsa 67-6149 Vintage A7 A10 A50 A65 Rear Brake Drum With Sprocket 42t (48.3% similar) This item had some surface rust and the flash makes it look orange. The teeth are in good shape with minimal wear. The only way to get fewer teeth on rear sprocket. QED wheel on swing arm a7 A10 A50 A65. This it is actually in good shape, although needs a cleaning and paint. Save money with combined...(May 2nd, 2025)
• Bsa 68-3303 Gearbox Layshaft A50 a65 1967-72 Used (39.2% similar) Holes are CLEAR.68-3303 Bsa used transmission lay shaft...(July 13th, 2025)
• Bsa 42-4281 A7 A10 Used Left Hand Bottom Gearbox Mounting Plate (39.6% similar) Used in good shape no damage some light rust and missing paint. See pictures for condition. Make an offer...(September 25th, 2025)
• Triumph Bsa W1322 New Cub T20 Rear Sprocket Bolt And Washer Set Part W1122 Of 8 (34.1% similar) New triumph cub T20 rear sprocket bolt washer set part # W1322 W1122 of 8, plated. Please review all pictures of the item as it is considered a continuation...(October 27th, 2025)
• Triumph Bsa 68-6120 A65 A50 Rear Sprocket And Bolts With Washers 47 Tooth 530 Chain Good Used Pt 68 (31.1% similar) Bsa A65 A50 rear sprocket bolts w/ washers 47 tooth 530 chain good used pt #68-6088,68-6095 and 68-6092 washers, original but in great shape. Rays Brit bits and more .:: listings by rage biz...(January 31st, 2026)
• Triumph Bsa 41-6090 B44 B25 Tr25w Rear Wheel Hub And Sprocket 56 Tooth 520 Chain Part (41.5% similar) Rays Brit bits and more .:: listings by rage biz. Bsa triumph B44 B25 TR25W rear wheel hub and sprocket 56 tooth 520 chain, part # 41-6090 cast in the drum...(February 3rd, 2026)
• Bsa 15-5664 Vintage B31 B33 Gearbox Chain Adjuster Bolt Nut (41.1% similar) Part #: Bsa 15-5664 for sale...(March 15th, 2026)