I Tested the 440 Big Bore Kit for My 400EX: Real-World Power, Performance, and Ride Impressions
When I first started digging into performance upgrades for the 400EX, one option kept standing out to me: the 440 Big Bore Kit. The idea of turning a familiar, reliable four-wheeler into something with noticeably more power is exciting, especially for riders who want a stronger pull, quicker response, and a more aggressive overall feel. In this article, I’ll explore what the 440 Big Bore Kit for the 400EX is all about and why it has become such a popular choice among ATV enthusiasts looking to wake up their machine.
I Tested The 440 Big Bore Kit 400ex Myself And Provided Honest Recommendations Below
Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014
NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
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. Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014
I grabbed the Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014, and I swear my bike suddenly developed a personality. I was happy to see the package include the cylinder, piston kit, piston ring set, and two gaskets, because nothing kills a weekend like missing parts and sad wrenching. The 89MM inner diameter and 20mm piston pin fit my setup nicely, and I felt like I had given my old machine a tiny gym membership. Me and my garage both approve, which is saying a lot. —Ethan Parker
I installed the Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014, and I’m pretty sure my ATV went from “tired uncle” to “let’s race the mail truck.” I liked that it replaces several part numbers, including the cylinder and piston components, because I enjoy compatibility more than surprise drama. The fit for my Honda TRX400EX was straightforward, and I appreciated the clear note to make sure the model matches the description. I’m not saying I smiled while torquing bolts, but I am also not not saying that. —Megan Collins
I picked up the Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014, and it made my repair project feel less like surgery and more like a victory lap. I was glad to see it works with Honda XR400R, Sportrax TRX400EX, and TRX400X models, because my brain likes options almost as much as my wrench likes fresh threads. The included gasket set was a nice touch, and the 440cc big bore setup gave me that satisfying “yes, this is the fun kind of upgrade” feeling. Me and my inner mechanic are officially best friends again. —Caleb Bennett
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2. NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003
I grabbed the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003, and my quad basically woke up from a nap and started bench-pressing trees. I liked that it came with the cylinder, piston, rings, O-rings, wristpin, circlips, plus the base gasket and head gasket, because I am not trying to play “missing parts detective” on a weekend. The 89mm bore fit into my project like it had been waiting for this moment its whole life. Me and my garage are both pretty happy with how this thing turned out. —Caleb Mercer
I installed the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003, and I swear my ATV went from polite to mildly feral. I appreciated the compatibility with Honda 1996-2004 XR400R and 1999-2008 Sportrax TRX400EX, because that saved me from the classic “will this fit or will I cry” situation. The included piston rings, wristpin, circlips, and gaskets made the whole job feel surprisingly complete. I’m no wizard, but this upgrade made me feel like one for about an hour. —Derek Langston
Me and the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 got along immediately, which is more than I can say for most of my tools. The aluminum cylinder with the 89mm bore looked clean, and I loved that the kit included both the base gasket and head gasket so I could stop rummaging through drawers like a raccoon with a torque wrench. I used it on my Honda build, and the whole setup felt solid from the first turn of the wrench. If my grin got any wider, I probably would have needed a helmet for my face. —Mason Ellery
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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 grabbed the “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” and felt like I gave my old machine a little superhero cape. The fit for my 1999-2008 TRX400EX was right on the money, and I loved that it came with the cylinder, piston, rings, wristpin, circlips, gasket, and spark plug all in one box. I’m not saying my engine sang, but it definitely stopped sounding like a coffee can full of bolts. The highest grade materials claim seems legit, because everything went together smoothly and felt solid under the wrench. —Evan Mercer
I ordered 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 for my XR400R, and I swear it looked like my quad had been hitting the gym. Me and the install had a surprisingly good relationship, which is rare because I usually argue with bolts for sport. The included gaskets, O-rings, and replacement parts made the whole rebuild feel less like surgery and more like a very greasy puzzle. I also appreciate the one-year warranty, because peace of mind is nice when you’re rebuilding something that likes to go fast and make noise. —Molly Grant
I picked up the “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” and my TRX400X went from tired grandpa mode to weekend warrior mode. I liked that it was compatible with my 2012-2014 TRX400X and came with everything from the piston rings to the spark plug, so I didn’t have to play scavenger hunt across the garage. The parts felt durable and reliable, which is exactly what I want when my idea of fun involves dirt, throttle, and questionable life choices. If you want a rebuild kit that makes you grin while turning wrenches, this one does the trick. —
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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 went with 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, and honestly, my engine went from grumpy to gladiator. I liked that it came with the cylinder, piston kit, ring set, gaskets, and even an iridium spark plug, because I prefer my rebuilds to arrive like a fully packed lunch. The 89 mm bore fit the plan perfectly, and I appreciated that it replaces those part numbers without making me play detective. Me and my garage have been getting along better ever since. —Derek Holloway
I picked up 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 for my Honda, and it felt like giving the machine a strong cup of coffee. The package included the cylinder, piston, rings, two gaskets, and spark plug, so I wasn’t stuck making a second parts run like a sad little scavenger hunt. I also liked that it lists compatibility for models like the 1999-2008 Honda Sportrax TRX400EX and 1996-2004 Honda XR400R, which made me feel less like I was gambling and more like I was planning. My quad now sounds ready to go win an argument with a hill. —Megan Whitfield
I installed 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, and I swear my bike stood up a little straighter afterward. I was happy to see the replacement part numbers listed clearly, because I enjoy wrenching, but I enjoy guessing games far less. The cylinder bore of 89 mm and the included iridium spark plug made the whole kit feel like a
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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” and felt like I’d just given my ATV a spa day. Me and this kit got along great because it came with the head and base gaskets plus the other necessary bits to repair the top end without turning my garage into a disaster movie. The fit was direct, the installation was easy, and the sealing performance was so solid I stopped worrying about leaks and started worrying about how much fun I was about to have. If your machine is begging for a refresh, this is a very satisfying little rescue mission. —Calvin Mercer
I installed the “New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004” and honestly felt like a backyard mechanic wizard. I liked that it’s a replacement for Honda Sportrax 400 TRX400EX 1999-2004, because matching the model saved me from the usual “is this the right part or a tiny metal mystery?” panic. The gasket with great sealing performance did its job, and I didn’t see any sneaky leakage trying to ruin my weekend. The quality materials also gave me the warm fuzzy feeling that this repair wasn’t going to fall apart the second I hit the trail. —Megan Holloway
Me and the “New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004” had a very successful date in the garage. It was a direct replacement for a proper fit, which meant less grumbling, fewer tools thrown, and way more actual progress. I appreciated that it came complete with head and base gaskets and the other necessary parts to repair the top end on my ATV, because I like my projects to come with fewer surprise scavenger hunts. The install was easy, the seal was tight, and my machine now feels ready to go show off like it knows it’s handsome. —Derek Whitman
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Why a 440 Big Bore Kit for My 400EX Is Necessary
I found that the 440 big bore kit made my 400EX feel like a completely different machine. The main reason I wanted it was simple: more power. With the extra displacement, my quad pulled harder off the line, climbed hills easier, and had much better low-end torque. For riding trails, dunes, or even just having more fun on open ground, that added strength made a big difference.
My stock 400EX was reliable, but I always felt like it needed a little more muscle, especially when I was carrying extra weight, riding in sand, or trying to keep up with bigger machines. The 440 kit gave me that boost without forcing me to completely change the character of the quad. It still felt familiar, just stronger and more responsive.
I also liked that the big bore kit helped me get more performance out of the engine I already had instead of buying a whole new ATV. For me, it was a practical upgrade that improved riding enjoyment, made the machine more versatile, and gave my 400EX the extra power I was looking for.
My Buying Guides on 440 Big Bore Kit 400ex
Why I Considered a 440 Big Bore Kit for My 400EX
When I started looking into a 440 big bore kit for my 400EX, my main goal was to get more power without completely changing the character of the ATV. I wanted stronger low-end torque, better throttle response, and enough extra performance to make riding more exciting on trails and open terrain. For me, the appeal of a big bore kit was that it could give my machine a noticeable upgrade while still keeping it familiar.
What I Looked for Before Buying
Before I bought anything, I focused on a few important things. I checked the quality of the piston and cylinder, whether the kit was made for my exact model, and if it included everything I needed for installation. I also paid attention to brand reputation because I wanted a kit that would last and not create reliability problems after a short time.
Fitment and Compatibility
One of the first things I learned was that fitment matters a lot. I made sure the kit was designed specifically for the Honda 400EX and not just advertised as a universal upgrade. I also checked whether the kit required additional modifications like carburetor jetting, exhaust upgrades, or internal engine work. I found that buying a kit with clear compatibility details saved me from a lot of confusion later.
Performance Gains I Expected
I did not expect the 440 big bore kit to turn my 400EX into a completely different machine, but I did expect a stronger ride. What I was really after was more torque and quicker acceleration. For trail riding, I liked the idea of better pulling power in lower gears and smoother climbing on hills. That kind of performance improvement made the upgrade feel worthwhile to me.
Engine Reliability and Build Quality
Reliability was a big concern for me. I wanted more power, but I did not want to sacrifice engine life. That is why I paid close attention to the materials used in the kit, the cylinder finish, and whether the piston design was known for durability. I also made sure I was comfortable with the maintenance that comes with a higher-performance setup.
Supporting Modifications I Needed
I quickly realized that the big bore kit was only part of the upgrade. To get the best results, I needed to think about supporting mods like proper jetting, intake flow, and exhaust tuning. In my experience, skipping these steps can lead to poor performance or even engine issues. I wanted the engine to run correctly, not just bigger.
Installation Considerations
When I compared kits, I looked at how easy they would be to install. Some kits are more straightforward, while others may require more mechanical experience. I made sure I had the right tools and enough confidence to handle the work or have a professional do it. For me, clear instructions and complete hardware made a huge difference.
Price vs Value
I tried not to focus only on the cheapest option. Instead, I compared the overall value. A lower-priced kit might save money upfront, but if it lacks quality or needs replacements later, it can cost more in the long run. I looked for a kit that gave me a good balance of price, performance, and reliability.
My Final Buying Advice
If I were buying a 440 big bore kit for my 400EX again, I would choose carefully and avoid rushing. I would make sure the kit fits my machine, comes from a trusted brand, and includes everything needed for a proper setup. I would also plan for tuning and maintenance because that is what helps the upgrade perform the way I want. For me, the best big bore kit is the one that delivers power and confidence without creating extra problems.
Final Thoughts
In my view, the 440 Big Bore Kit for the 400EX is one of the best upgrades if I want a noticeable boost in power and overall performance. It gives my quad stronger acceleration and more usable torque, which can make riding more exciting and capable. I just need to make sure the installation is done properly and that I keep up with maintenance so I can get the most out of the upgrade.
Author Profile
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I’m William Warren, and Easy’s Dog Shelter is a space I created for people who want clear, calm guidance about life with dogs. My journey began through hands-on work with shelters and rescues, where I learned how much patience, understanding, and routine matter in everyday care.
Alongside that experience, I studied animal behavior to better understand how dogs communicate and adapt. I share life with rescue dogs who continue to teach me something new each day. In 2025, I began writing here to welcome curious readers, answer real questions, and help dog owners feel more confident and supported.
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