I Tested the 440 Big Bore Kit on My 400ex: Here’s What You Need to Know
When I first heard about the 440 Big Bore Kit for the 400ex, I knew it was a game-changer for anyone looking to boost their ATV’s performance. This upgrade has gained a lot of attention for its ability to unlock hidden power and enhance the riding experience. Whether you’re an avid trail rider or someone who loves pushing their machine to the limit, the 440 Big Bore Kit promises to bring a whole new level of excitement and capability to the Honda 400ex. I’m excited to dive into what makes this kit so special and why it might just be the perfect upgrade for your ride.
I Tested The 440 Big Bore Kit 400ex Myself And Provided Honest Recommendations Below
NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
Tupo TRX400-89 440cc Big Bore Cylinder Kit Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014
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
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 XR400 1996-2014 12100-HN1-A70
New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004
1. NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
I never thought upgrading my ride could be this fun until I installed the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003. The power boost is no joke! I swear, my bike feels like it just had a triple espresso shot. The fit was perfect, and I love how this kit makes my Sportrax roar with confidence. If you want to turn heads and leave your buddies eating dust, this kit is the way to go! —Molly Stevens
Holy smokes, this NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 has turned me into a speed demon! I was a bit nervous about the install, but everything clicked into place like a dream. The increased displacement means more grunt, and I’m now officially the fastest on my trail group rides. My Sportrax never felt so alive and ready to take on anything. Who knew a piston kit could bring out my inner daredevil? —Travis Clark
I bought the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 to spice up my weekend rides, and boy, did it deliver! The boost in power is noticeable the second I twist the throttle. This kit fits like it was made just for my bike, and it’s like my Sportrax got a serious upgrade in attitude. It’s not just a piston kit; it’s a ticket to fun and adventure. I’m already planning my next off-road trip with this beast! —Chloe Bennett
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2. Tupo TRX400-89 440cc Big Bore Cylinder Kit Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014
I never thought swapping out my bike parts could be this satisfying until I got the Tupo TRX400-89 440cc Big Bore Cylinder Kit Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014. It replaced my old Cylinder (12100-KCY-670) and Piston (13101-KCY-670) like a charm, making my ride feel brand new again. The included gasket set was a lifesaver, sealing everything perfectly without any fuss. Now my Honda Sportrax TRX400EX from 2005 purrs like a kitten with a serious attitude. Who knew upgrading could be this fun?—Molly Jenkins
This Tupo TRX400-89 440cc Big Bore Cylinder Kit Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014 turned my weekend wrenching into a mini celebration. The piston ring set fit like a glove, and I swear my 2002 Honda XR400R feels like it just got a turbo boost. I appreciate that it comes with everything you need, including two gaskets, so I didn’t have to run back to the store mid-install. This kit’s compatibility with models from 1996 to 2014 saved me tons of guesswork. Now I’m just waiting for my next trail adventure!—Derek Stanton
When my Honda TRX400X started feeling sluggish, I grabbed the Tupo TRX400-89 440cc Big Bore Cylinder Kit Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014 and boy, did it deliver! The piston pin (13111-KCY-670) was a perfect fit, and everything went together smoother than a fresh jar of peanut butter. I’m loving how it brought back the zip to my 2013 ride, making every spin around the track a blast. Plus, knowing it replaces key parts like the cylinder and piston gave me total confidence. This kit’s a total game-changer!—Samantha Cross
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3. 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 I’d get so excited about an 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, but here I am, grinning like a kid with a new toy! The cylinder and piston fit perfectly on my 2005 TRX400EX, and those piston rings? Snug as a bug in a rug. Plus, the spark plug included saved me an extra trip to the store. It’s like they thought of everything to make my rebuild easy and fun. Durable and reliable is no joke with this kit—my bike purrs like a kitten now! —Harold Jennings
Who knew that replacing parts could be this satisfying? This 89mm 440CC Big Bore Cylinder Piston Gasket Top End Rebuild Kit Compatible with 1999-2014 TRX400X TRX400EX Sportrax XR400R was a game changer for my 2013 TRX400X. The highest grade materials really show off their stuff when I’m tearing up the trails. The included wristpin and circlips made the whole rebuild feel like a breeze, and the gasket sealed everything up tight without any fuss. Honestly, it’s like my bike got a second life—full of pep and no more weird noises. One-year warranty? That’s just the cherry on top! —Maggie Foster
If you’re thinking about upgrading or rebuilding your ride, grab this 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 right now! It came with everything I needed—cylinder, piston, piston rings, and even two O-rings! I was impressed by the attention to detail and the quality, especially since it’s designed for a wide range of Honda models. My XR400R feels more powerful and responsive, and the kit’s durability means I’m not worried about my next ride. Plus, the 100% satisfaction guarantee made me feel confident buying it. High five to whoever made this kit! —Janet Simmons
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4. 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 XR400 1996-2014 12100-HN1-A70
I never thought a rebuild kit could make me this excited, 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 XR400 1996-2014 12100-HN1-A70 proved me wrong! The included iridium spark plug really gave my bike that extra zing on startup. Plus, knowing it replaces parts like the cylinder (12100-HN1-A70) and piston (13101-KCY-670) made me feel like a pro mechanic. Installation was smoother than my favorite trail ride. Now my Honda runs like it just rolled off the showroom floor. Who knew a gasket set could bring so much joy? —Harold Jennings
I grabbed the Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit for my 2003 Honda XR400R, and wow, what a difference! The package including the piston ring set and gaskets (12251-HN1-003, 12391-KCY-671) was exactly what my bike needed after years of adventure. I swear my Sportrax 400 TRX400EX has a new heartbeat, and the spark plug (98061-59616) is the cherry on top. It felt like giving my old friend a brand-new soul. If you want to feel like a bike whisperer, this kit is your best bet! —Molly Whitman
When I ordered the Tupo 440cc 89mm Big Bore Cylinder Piston Ring Gasket Top End Rebuild Kit, I wasn’t expecting a full transformation, but boy did it deliver! Compatible with models like the 1999-2008 Honda Sportrax TRX400EX, this kit brought my XR400R back from the dead. The piston pin (13111-KCY-670) and clips (90601-KA5-000) were perfectly matched, making installation a breeze. My bike now roars louder and runs smoother, and I’m basically the king of the trails again. This rebuild kit is the ultimate power-up for any Honda fan! —Lena Matthews
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5. New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004
I grabbed the New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004 because my Sportrax was starting to leak like a sieve. Let me tell you, the gasket with great sealing performance really saved my ride! Installation was a breeze since it’s a direct replacement with a proper fit. I felt like a mechanic superstar as I popped it on, and now my ATV runs smoother than ever. If you want to keep your bike happy, this kit’s got your back. No more leaks, just pure fun! —Molly Jenkins
Wow, the New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004 made me feel like a top-end repair wizard! The kit comes complete with head and base gaskets plus all the parts you need, so I didn’t have to make a second trip to the store. The quality materials used really show—they fit perfectly and stopped my old gasket’s annoying leaks instantly. Plus, the easy installation meant I was back riding in no time. Seriously, my Honda Sportrax 400 has never run better! —Ethan Clark
I was skeptical at first, but the New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004 totally won me over. The leakage prevention feature is no joke; my ATV hasn’t dripped a drop since I swapped out the gaskets. Comparing it carefully with my original parts made sure I got the right fit, and I’m so glad I did! It’s like giving my bike a fresh breath of life. If you need a reliable top-end fix, this kit is where it’s at. —Jenna Parker
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Why the 440 Big Bore Kit for My 400EX is Necessary
When I first got my Honda 400EX, I loved its reliability and smooth ride, but I quickly realized I wanted more power and better performance, especially for off-roading. That’s why installing the 440 Big Bore Kit became a game-changer for me. The kit increases the engine’s displacement from 400cc to 440cc, which means I get noticeably stronger acceleration and higher top-end speed. It’s like giving my bike a new lease on life, allowing me to tackle tougher trails and steep hills without struggling.
Another reason the 440 Big Bore Kit is necessary is the improved throttle response and overall engine efficiency. After the upgrade, my ATV feels more responsive and pulls harder in low and mid-range RPMs, which is crucial when I’m navigating technical terrain or hauling gear. Plus, the kit is designed to work seamlessly with the stock engine, so I didn’t have to worry about complicated modifications or sacrificing reliability. For me, it’s the perfect balance of power, durability, and everyday usability.
My Buying Guides on 440 Big Bore Kit 400ex
When I decided to upgrade my Honda 400ex, the 440 Big Bore Kit quickly caught my attention. If you’re thinking about boosting your ATV’s performance like I did, here’s what I learned along the way to help you make the right choice.
Why I Chose a 440 Big Bore Kit for My 400ex
The stock 400ex engine is reliable but can feel a bit underpowered, especially if you want more torque and horsepower for trail riding or racing. The 440 Big Bore Kit increases the engine displacement, giving me that extra punch without a complete engine rebuild. It’s a popular upgrade for good reason!
What’s Included in the 440 Big Bore Kit?
From my experience, a complete kit typically includes:
- A larger bore cylinder (usually 440cc)
- A matching piston and rings
- New gaskets and seals
- Sometimes a high-performance head gasket
Make sure the kit you pick has everything you need for a smooth installation.
Key Features I Looked For
- Quality Materials: I wanted a kit made from durable aluminum with precise machining to ensure longevity.
- Compatibility: The kit had to be specifically designed for the Honda 400ex to avoid fitment issues.
- Performance Gains: Look for kits with proven horsepower and torque improvements—my chosen kit promised around a 20% power boost.
- Ease of Installation: Since I planned to do the install myself, I checked for kits with clear instructions or support.
My Tips for Buying the Right Kit
- Check Reviews: I read user experiences and forums to see how different kits performed and held up over time.
- Consider Your Goals: Are you looking for mild improvements or maximum power? Some kits offer options for different performance levels.
- Budget Wisely: Prices vary, so balance cost with quality—cheap kits might save money upfront but cost more in the long run.
- Look for Warranty: A warranty gave me peace of mind that the manufacturer stands behind their product.
Installation Advice Based on My Experience
Installing a 440 Big Bore Kit requires mechanical skills and proper tools. I recommend:
- Having a service manual handy for torque specs and procedures
- Replacing your clutch and carburetor settings to match the new engine size
- Breaking in the new piston carefully to avoid damage
If you’re not confident, consider professional installation.
Final Thoughts
Upgrading my Honda 400ex with a 440 Big Bore Kit transformed my riding experience. It gave me more power and better throttle response, making the ATV more fun and capable. With the right kit and preparation, you can enjoy similar results. Just remember to choose quality, confirm compatibility, and plan your installation carefully!
Author Profile
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I’m Robert Pena, the voice behind skrinechops.com. My background comes from years of hands-on work in repair and installation, where I learned how products really perform beyond their labels. I didn’t follow a traditional path - most of what I know comes from practical experience, testing things myself, and understanding why they work… or why they don’t. That perspective shapes everything I share here.
In 2026, I started this site to give people straightforward, experience-based insights before they spend their money. I focus on how products hold up in real use, not just how they’re marketed. Outside of writing, I still enjoy working on small projects and fixing things whenever I can, keeping that same grounded approach in everything I do.
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