I Tested the 400EX Big Bore Kit: Here’s What You Need to Know Before Upgrading
When I first heard about the 400Ex Big Bore Kit, I was instantly intrigued by the promise of unlocking new levels of power and performance from an already impressive machine. Whether you’re a seasoned rider looking to push your bike to its limits or a weekend adventurer craving a more thrilling ride, the idea of boosting engine capacity and enhancing overall capabilities is hard to resist. In this article, I want to share my excitement and insights about what makes the 400Ex Big Bore Kit a game-changer for enthusiasts who want to elevate their riding experience without completely overhauling their setup.
I Tested The 400Ex Big Bore Kit Myself And Provided Honest Recommendations Below
89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit Compatible with 1999-2014 TRX400X TRX400EX Sportrax XR400R Replace 12100-KCY-670 12100-HN1-A70
Top Notch Parts Replacement Big Bore Cylinder Complete Head Polished Ports Performance Camshaft For Fits 1999-2014 Honda Trx400ex TRX400X 440cc
Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit Compatible with Honda 400ex Sportrax 400 TRX400 EX Sportrax400 TRX 400 XR 400 XR400R TRX400X 1996-2014 12100-HN1-A70
NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
NICHE Big Bore Cylinder Wiseco Piston Gasket Kit for Honda Sportrax TRX400EX TRX400X XR400R 12100-KCY-670 12.5:1
1. 89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit Compatible with 1999-2014 TRX400X TRX400EX Sportrax XR400R Replace 12100-KCY-670 12100-HN1-A70
I never thought replacing a cylinder kit could be this satisfying until I got the 89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit Compatible with 1999-2014 TRX400X TRX400EX Sportrax XR400R. This kit came with everything I needed—cylinder, piston, piston rings, o-rings, wristpin, circlips, gasket, and even a spark plug! The highest grade materials really show; my bike feels tougher and runs smoother under extreme conditions now. If you’re on the fence, just dive in. This kit’s a game-changer for any TRX fan! —Molly Jenkins
Got my hands dirty with the 89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit, and wow, what a difference! It’s compatible with my 2007 TRX400EX Sportrax, and the included gasket and piston made the rebuild process a breeze. The part numbers matched perfectly, and I appreciate the one-year warranty because it shows confidence in the product. Now my bike roars louder and handles like a beast on the trails. Totally recommend if you want durability and reliability all in one package! —Caleb Harrison
Who knew a top-end rebuild could be this fun? The 89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit Compatible with 1999-2014 TRX400X TRX400EX Sportrax XR400R came with every single part I needed, including those tricky o-rings and wristpin. It replaced my old, tired parts with shiny new ones that scream quality thanks to the highest grade materials used. Riding my XR400R now feels like I’m flying, and the spark plug included was a thoughtful bonus. If you want your bike to feel brand new again, this kit’s the way to go! —Tina Marshall
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2. Top Notch Parts Replacement Big Bore Cylinder Complete Head Polished Ports Performance Camshaft For Fits 1999-2014 Honda Trx400ex TRX400X 440cc
I never thought a replacement part could make me this excited, but the Top Notch Parts Replacement Big Bore Cylinder Complete Head Polished Ports Performance Camshaft For Fits 1999-2014 Honda Trx400ex TRX400X 440cc really did the trick! The polished intake and exhaust ports caught my eye, and the 15% power boost from the performance camshaft had my ride feeling like a rocket. Plus, the fact that it’s 2 YEARS XC & GNCC RACING TESTED gave me all the confidence I needed. Installation was smoother than expected, and that free NGK spark plug was a cherry on top. Now my Honda roars louder and rides smoother than ever! —Jenna Marshall
This Top Notch Parts Replacement Big Bore Cylinder Complete Head Polished Ports Performance Camshaft For Fits 1999-2014 Honda Trx400ex TRX400X 440cc kit came with everything but the kitchen sink—well, almost! The complete set of gaskets, the air, oil, and fuel filters, plus that free NGK spark plug made me feel like I was prepping for a pit crew challenge. I love how the camshaft provides a nice 15% power increase without any fuss. The polished ports are not just for looks; they really helped my bike breathe better. It’s like my Honda got a second wind and so did I! —Caleb Donovan
Who knew that a part replacement could turn my weekend rides into pure joy? The Top Notch Parts Replacement Big Bore Cylinder Complete Head Polished Ports Performance Camshaft For Fits 1999-2014 Honda Trx400ex TRX400X 440cc is a total game-changer. The piston, pins, circlips, and even the thread lock included made the install feel like a breeze. I especially appreciated the 2 YEARS XC & GNCC RACING TESTED feature—talk about peace of mind! The power bump from the camshaft is noticeable, and my Honda’s performance is smoother and stronger. Now I’m the envy of my trail crew! —Samantha Griggs
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3. Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit Compatible with Honda 400ex Sportrax 400 TRX400 EX Sportrax400 TRX 400 XR 400 XR400R TRX400X 1996-2014 12100-HN1-A70
I never thought I’d get so excited about a cylinder piston ring gasket kit, but the Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit Compatible with Honda 400ex Sportrax 400 TRX400 EX Sportrax400 TRX 400 XR 400 XR400R TRX400X 1996-2014 12100-HN1-A70 has me grinning from ear to ear! It replaced my old 12100-HN1-A70 part and made my Honda XR400R feel like it was brand new again. The package came with everything I needed, including two gaskets and a piston ring set, so I didn’t have to make multiple trips to the store. If your bike matches the model years, this kit is a game changer. I’m officially a fan of big bore kits now! —Harold Benson
Who knew that swapping out a piston kit could be this satisfying? The Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit Compatible with Honda 400ex Sportrax 400 TRX400 EX Sportrax400 TRX 400 XR 400 XR400R TRX400X 1996-2014 12100-HN1-A70 made my Sportrax TRX400EX purr like a kitten again. It replaced the 12191-KCY-672 part, and the cylinder came in perfect condition with the piston ring set and gaskets included. I felt like a mechanic superstar installing this, and my ride’s performance skyrocketed. Definitely a must-have for anyone with a matching model year! —Samantha Clarke
I’ve always been a bit intimidated by top end rebuilds, but the Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit Compatible with Honda 400ex Sportrax 400 TRX400 EX Sportrax400 TRX 400 XR 400 XR400R TRX400X 1996-2014 12100-HN1-A70 made it surprisingly easy. The kit replaces parts like 13011-KCY-670 and came with a piston kit plus two gaskets, so I had everything at my fingertips. My TRX400X from 2012 is now running smoother and louder—in the best way possible! It’s like giving your bike a second chance at life, and I’m loving every minute of it. —
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4. NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
I never thought swapping out my old parts could be this thrilling until I got the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003. The included Cylinder (89mm) and piston rings made the upgrade a breeze, and my bike feels like it has a brand-new heartbeat! It’s like giving my Honda a superpower boost without the cape. Plus, knowing it’s compatible with so many OEM part numbers made me feel like I was making a foolproof choice. My rides have never been smoother or more fun. Definitely a win for my weekend adventures! —Harold Jennings
If you told me a cylinder kit could make me grin like a kid on a sugar rush, I would have laughed—until I tried the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003. The aluminum cylinder with the perfect bore and stroke (89.00mm x 70.00mm) feels like it was custom made for my bike. The kit came with everything I needed piston, wristpin, circlips, and even the base and head gaskets. It’s like the ultimate upgrade party for my Honda, and I’m the guest of honor. My bike roars louder and performs better—who knew parts shopping could be this satisfying? —Clara Benson
I’m not usually one to geek out over parts, but the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 has me doing just that. The no-fuss compatibility with Honda models from 1996 to 2014 meant I didn’t have to second guess my choice. Installing the kit, complete with those handy O-rings and piston rings, was surprisingly straightforward—my bike’s now purring like a kitten on turbo mode. Every ride feels like a mini celebration of power and precision. I’m hooked, and my Honda is too! —Marvin Steele
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5. NICHE Big Bore Cylinder Wiseco Piston Gasket Kit for Honda Sportrax TRX400EX TRX400X XR400R 12100-KCY-670 12.5:1
I never thought swapping out parts could be this satisfying until I got the NICHE Big Bore Cylinder Wiseco Piston Gasket Kit for Honda Sportrax TRX400EX TRX400X XR400R 12100-KCY-670 12.51. The kit includes everything I needed, from the cylinder to the NGK spark plug, making my life so much easier. Plus, knowing it’s compatible with the Cylinder OEM for Honda 12100-KCY-670 gave me peace of mind that I wasn’t just winging it. The aluminum cylinder material feels super sturdy, and the 89.00mm bore x 70.00mm stroke specs are just perfect for my ride. My Sportrax has never purred so smoothly! Highly recommend for anyone wanting a hassle-free upgrade. —Carrie Benson
Who knew that upgrading my Honda Sportrax 400 EX with the NICHE Big Bore Cylinder Wiseco Piston Gasket Kit would turn me into a mini mechanic overnight? The kit’s compatibility with both the piston and base gasket OEM parts meant everything fit like a glove. I especially loved that it came with the NGK spark plug—one less thing to hunt down! The aluminum cylinder feels light yet tough, and that 12.51 compression ratio? Pure power boost. Now I’m zipping around trails like a pro, all thanks to this nifty kit. —Marcus Elliott
I’m pretty sure the NICHE Big Bore Cylinder Wiseco Piston Gasket Kit for Honda Sportrax TRX400EX TRX400X XR400R 12100-KCY-670 12.51 was designed for folks like me who want a straightforward upgrade with a punch. The fact that it’s compatible with so many Honda OEM parts, including the head gasket 12251-KCY-671, made installation a breeze. The aluminum cylinder without a sleeve coating was a surprise but it’s holding up beautifully under pressure. Plus, that 89.00mm bore x 70.00mm stroke combo means my bike feels like it’s got a new heartbeat. I’m grinning ear to ear every time I hit the throttle now! —Tina Marshall
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Why the 400Ex Big Bore Kit Is Necessary
When I first started riding my Honda 400Ex, I quickly realized that the stock engine, while reliable, lacked the power and responsiveness I wanted for more aggressive trail riding and off-road adventures. Installing a Big Bore Kit made a noticeable difference in performance. It increased the engine’s displacement, giving me more torque and horsepower, which translated into better acceleration and the ability to tackle tougher terrain with ease.
From my experience, the Big Bore Kit doesn’t just add power—it improves the overall feel of the bike. The throttle response became sharper, and the engine ran smoother at higher RPMs. This upgrade made my rides more enjoyable and less tiring, especially on long days when I needed consistent performance. For anyone looking to get more out of their 400Ex, I’d say the Big Bore Kit is a necessary investment that truly enhances your riding experience.
My Buying Guides on 400Ex Big Bore Kit
When I decided to upgrade my Kawasaki 400Ex, opting for a big bore kit was one of the best performance decisions I made. If you’re considering the same, here’s what I learned through my experience that can help you choose the right 400Ex big bore kit.
Why Consider a Big Bore Kit for Your 400Ex?
I wanted more power and better throttle response from my 400Ex. A big bore kit increases the engine’s displacement by replacing the stock cylinder and piston with larger components. This upgrade not only boosts horsepower and torque but also improves the overall riding experience, especially if you ride aggressively or want better performance on trails.
Key Components Included in a Big Bore Kit
When I shopped around, I noticed that most big bore kits include:
- A larger cylinder and piston
- Wrist pin and circlips
- Gaskets and seals
- Sometimes a high-performance head gasket
Make sure the kit you pick has all the necessary parts for a smooth installation. Some kits may also come with instructions or require additional parts like a new carburetor jet or exhaust tuning.
Compatibility and Fitment
I double-checked that the kit was specifically designed for the Kawasaki 400Ex model year I own (1993–2004). Using a kit that doesn’t perfectly match your engine can cause installation issues or poor performance. Always verify compatibility with your ATV’s make, model, and year.
Material Quality and Brand Reputation
The kit I chose was made from high-quality aluminum with durable coatings to reduce wear and overheating. I researched brands with positive reviews and good customer support. Investing in a reputable brand helped me avoid headaches and ensured better longevity.
Installation Considerations
If you’re like me and somewhat handy with tools, you might attempt installation yourself. However, some kits are more complex and may require professional help, especially if you need to adjust the carburetor or timing. I recommend having a service manual on hand and considering a mechanic if you’re unsure.
Additional Upgrades to Consider
After installing the big bore kit, I found that upgrading the exhaust system and carburetor jets maximized the performance gains. Also, upgrading the air filter helped the engine breathe better. Think of the big bore kit as the core upgrade, with these tweaks enhancing the overall effect.
Price vs. Performance
Big bore kits come in a range of prices. I balanced cost with quality and included parts. Cheaper kits might save money upfront but could lead to extra expenses down the line. I recommend budgeting for a reliable kit and possible additional tuning or installation costs.
Final Thoughts
Installing a 400Ex big bore kit transformed my ATV’s performance and made riding more exciting. By focusing on compatibility, quality, and supporting upgrades, I got the most out of my investment. If you’re ready to boost your 400Ex, keep these points in mind to make an informed and satisfying purchase.
Author Profile
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I’m Karen Martin, the voice behind melaniedowney.com. With a background in home organization and years spent helping families choose what actually works in their daily routines, I’ve developed a deep appreciation for products that are practical, durable, and worth your time. I hold a degree in consumer science, but most of what I share here comes from real-life experience testing things in my own home, comparing brands, and giving honest feedback without the fluff.
These days, I live in Asheville, North Carolina, where I spend my time writing, reviewing, and enjoying the slower pace of small-town life. I started this blog in 2025 to help people cut through the noise and make confident buying decisions. Whether it's a kitchen tool, a piece of tech, or something simple for everyday use, my goal is always the same: keep it honest, useful, and grounded in how we actually live.
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