CR85 Big Bore Kit: Power & Performance

Honda CR85 Big Bore Kit: Unlocking More Power

The Honda CR85 is a formidable machine in the world of motocross, designed for young riders looking to hone their racing skills. However, for those seeking an extra edge on the track, a big bore kit can significantly enhance performance. This article delves into what a typical CR85 105cc Big Bore kit includes, the technical aspects of the upgrade, and the benefits it offers to riders. We'll explore the components, the engineering behind the power gains, and considerations for installation.

What is a CR85 Big Bore Kit?

A big bore kit is essentially an upgrade designed to increase the engine's displacement, leading to more power and torque. For the Honda CR85, a popular modification is the 105cc kit, which typically involves a larger bore cylinder and a matching piston. This modification aims to provide a broader and more potent powerband, crucial for competitive racing.

What Does a CR85 105cc Big Bore Kit Include?

A comprehensive CR85 105cc Big Bore kit, specifically designed for the 2005 and later models, is a meticulously engineered package aimed at delivering a significant performance boost. Here's a breakdown of what you can expect:

Engine Porting and Finishing: The Key to Performance

Beyond the core components, the porting and finishing of the cylinder are critical for unlocking the full potential of a big bore kit. In a performance-oriented kit like this:

• Exhaust Port Enlargement and Polishing: The exhaust port is enlarged and polished. This allows for a quicker and more efficient expulsion of burnt gases, significantly boosting mid-range and top-end power without sacrificing low-end grunt.
• Transfer Port and Intake Port Refinement: Rough casting flaws are removed from the transfer and intake ports. These ports are then slightly enlarged and finished with a glass beaded surface. This finish aids in preventing fuel/air separation, ensuring a more homogenous mixture enters the combustion chamber, and promotes better airflow.
• Polished Combustion Chamber: The combustion chamber of the cylinder head is also polished. This smooths the flow of the air-fuel mixture into the chamber and can help prevent carbon build-up, contributing to consistent performance.

Nikasil Plating: A Superior Surface

The use of Nikasil plating on the cylinder is a hallmark of a high-quality big bore kit. Nikasil is a nickel-silicon composite coating that offers several advantages over standard cylinder liners:

• Durability: Nikasil plating is significantly harder and more wear-resistant than cast iron or chrome plating, leading to a longer cylinder life.
• Reduced Friction: It provides a smoother surface, reducing friction between the piston and cylinder wall. This translates to more efficient power transfer and potentially higher peak horsepower.
• Heat Dissipation: Nikasil plating is an excellent conductor of heat, helping to keep the cylinder cooler under stress. This is vital for preventing engine damage, especially in high-performance applications.
• Thickness: The kit specifies a Nikasil plating thickness of a minimum of .004 inches, which is substantially thicker than the stock plating (.001"-.002"). This increased thickness adds to the overall robustness and lifespan of the cylinder.

Performance Gains and AMA Legality

The combination of increased displacement and meticulous porting results in substantial performance gains across the powerband. The larger bore inherently adds power throughout the rev range, while the porting enhancements specifically target mid-range and top-end improvements. Crucially, this specific kit is noted as being AMA legal for the Supermini class, making it a viable option for racers competing in this category.

Pricing and Core Exchange

The pricing structure for this kit typically involves a core exchange to manage costs:

• $600.00: This price is applicable if you send in your old cylinder and head for the core charge. This is a common practice where the supplier reuses or reconditions your original parts.
• $900.00: This price includes a $300.00 refund once your old cylinder and head are received for the core charge. This offers flexibility if you need the kit immediately and will return your old parts later.

For immediate shipping, payment via credit card is often preferred. A common method is to charge the full $600.00 and place a $300.00 "authorization" hold on your card. This hold does not charge your card but reserves the funds. The authorization is typically voided once your old cylinder and head are received.

Replacement Parts Availability

It's important to know that specialized components like custom pistons and rings are available for these kits. A replacement piston for this kit might cost around $150.00, with replacement rings priced at approximately $12.00. It's reassuring that suppliers usually keep these critical parts in stock to support the kit long-term.

Shipping

Often, suppliers will offer free UPS ground shipping on these kits, which adds to the overall value proposition.

Honda CR85 Engine Specifications (Stock)

To understand the impact of the big bore kit, it's helpful to look at the stock specifications of the Honda CR85 engine:

Is a Big Bore Kit Right for You?

A big bore kit is an excellent modification for experienced riders who want to maximize their CR85's performance for racing. However, it's important to consider the following:

• Riding Skill: The stock CR85 already has a potent, 'on/off' powerband due to its lack of a power valve and its racing-tuned nature. A big bore kit can make the power delivery even more aggressive. This is generally not recommended for beginner riders who may struggle to manage the increased power and narrower powerband.
• Maintenance: High-performance engines, especially modified two-strokes, require diligent maintenance. Regular top-end rebuilds, proper jetting, and the correct fuel-oil mixture are essential to ensure longevity and consistent performance.
• Suspension: The stock CR85 suspension is tuned for riders typically weighing between 70-100 lbs. If you are a heavier rider, you may need to consider upgrading the suspension springs and valving to match the increased power and potential for higher speeds.
• Chassis and Handling: While the CR85 is known for its light weight and nimble handling, increased power can put more stress on the chassis components. Ensure the bike's overall condition is sound to handle the added performance.

Frequently Asked Questions (FAQs)

Q1: Will a big bore kit make my CR85 faster?
Yes, a properly installed big bore kit will increase horsepower and torque, leading to faster acceleration and a higher potential top speed, especially when the engine is kept within its optimal RPM range.

Q2: Is the CR85 Big Bore kit difficult to install?
Installation requires mechanical proficiency, especially with engine work. It involves disassembling the top end of the engine, replacing the cylinder and piston, and ensuring all components are torqued correctly. If you are not experienced, professional installation is recommended.

Q3: How often will I need to rebuild the top end with a big bore kit?
With a performance-oriented big bore kit and aggressive riding, top-end rebuilds may be needed more frequently than stock. For expert riders on a track, this could be every 10-20 hours. Casual riders who maintain the bike well might get 100 hours or more.

Q4: Can I use a different brand of piston with this kit?
It's highly recommended to use the specified custom-made piston for the kit. Using a piston of the incorrect size or specification can lead to poor performance, engine damage, or premature failure.

Q5: What is the difference between the CR85R and CR85RB (Expert) models in relation to big bore kits?
The CR85RB (Expert) model typically comes with larger wheels and a slightly longer swingarm to accommodate taller riders. While the big bore kit itself is generally compatible, the suspension tuning and overall setup might differ due to the rider size and intended use.

Conclusion

The 105cc Big Bore kit for the Honda CR85 represents a significant upgrade for riders seeking to extract maximum performance from their machine. By increasing displacement and meticulously refining the engine's ports, this kit offers a substantial power boost that can make a real difference on the motocross track. Understanding the components, the engineering involved, and the maintenance requirements is key to successfully implementing this modification and enjoying the enhanced performance it provides.

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