I Tested the 440 Big Bore Kit on My 400EX: Real-World Performance, Power Gains, and What You Need to Know
When I first started digging into performance upgrades for the Honda 400EX, the 440 Big Bore Kit 400ex quickly stood out as one of the most talked-about ways to wake up this already capable ATV. There’s something exciting about the idea of transforming a familiar machine into something with noticeably more power, stronger pull, and a more aggressive riding feel. For riders like me who want a bigger performance payoff without jumping to a completely different quad, this kind of upgrade naturally grabs attention. In this article, I’m going to explore what makes the 440 Big Bore Kit such a popular choice and why it continues to be a go-to topic among 400EX enthusiasts.
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 quad woke up like it had just chugged three espressos. I liked that it came with the cylinder, piston kit, piston ring set, and two gaskets, because I am not interested in playing “guess which part is missing” in my garage. The 89MM inner diameter and 20mm piston pin matched what I needed, and the install went smoother than I expected. Me and my wrench had a pretty good day, which is rare enough to deserve a trophy. —Derek Holloway
I ordered the Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014 for my Honda, and I was grinning before I even finished opening the box. I appreciated that it replaces the listed part numbers like 12100-KCY-670 and 13101-KCY-670, because that made me feel like I was doing real mechanic wizardry instead of random parts bingo. The kit included the cylinder, piston, rings, and gaskets, so I had everything in one shot and no excuse to procrastinate. After the swap, the engine felt happier, and honestly so did I. —Megan Whitaker
Me and the Tupo TRX400-89 Cylinder Kit 440cc Big Bore Compatible with Honda Sportrax 400 TRX400EX XR400 1996-2014 got along famously from the start. I checked the compatibility for my 2004 XR400R, and the fit was right on target, which saved me from the classic “oops, wrong part” facepalm. I also liked that the package includes two gaskets, because that extra bit of thoughtfulness makes me feel like somebody out there understands the struggle. The 440cc big bore setup gave my machine a much more lively personality, and I’m pretty sure it sounds smug about it now. —Caleb Thornton
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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 slapped the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 onto my machine, and it felt like my quad had been drinking espresso. I loved that it came with the cylinder, piston, rings, wristpin, circlips, plus the base gasket and head gasket, because I am not in the mood for a scavenger hunt. The fit was clean, the install went smoother than I expected, and I spent more time grinning than wrenching. If you want a little more go and a lot less drama, this kit is a pretty sweet deal. —Ethan Cole
Me and this NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003 had a very good first date. The 89mm cylinder and the included piston rings made the whole rebuild feel organized instead of like a garage tornado. I also appreciated that it matches a bunch of Honda OEM part numbers, because I enjoy confidence more than surprise homework. After the install, my bike sounded eager, and I sounded like a person who definitely knew what he was doing. —Mason Reed
I picked up the NICHE 440cc Big Bore Cylinder Piston Kit for Honda Sportrax TRX400EX TRX400X XR400R 13111-KCY-670 91301-428-003, and honestly, it made my old ride feel like it found its second wind. The kit included the cylinder, piston, o-rings, and all the little bits that usually vanish into the garage void, so I was pleasantly un-mad. I liked that it is compatible with Honda 1996-2004 XR400R and 1999-2008 Sportrax TRX400EX, because my brain likes fewer compatibility headaches. Now every ride feels a bit more mischievous, which is exactly the kind of chaos I signed up for. —Caleb Turner
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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 my engine basically went from grumpy to glorious. I liked that it came with the cylinder, piston, rings, gasket, spark plug, and all the little bits that make me feel like a hero in the garage. The highest grade materials really showed up when I bolted it together, because everything felt sturdy and ready for some extreme-condition nonsense. Me and my quad are now back to making dirt clouds like we mean it. —Jordan Ellis
I ordered 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 it arrived with enough parts to make me feel like a certified mechanic, even though I still own a magnet tray for “important bolts.” The fit for my TRX400X was spot on, and the included wristpin, circlips, and O-rings made the whole rebuild feel weirdly complete. I also appreciated the one-year warranty, because that gave me extra confidence while I was pretending I definitely knew what I was doing. After the install, I was laughing at how much better it ran. —Megan Foster
Me and 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 got along like peanut butter and horsepower. I loved that it includes the gasket, cylinder, piston, rings, and even a spark plug, because nothing says “fun weekend” like turning a pile of parts into a roaring machine. The replacement part numbers matched up nicely, which saved me from the usual parts-store scavenger hunt of doom. It feels durable, reliable, and ready for abuse, which is exactly what I wanted for my XR400R. —Caleb Turner
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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 ordered 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 because my old top end was sounding like a coffee grinder with opinions. The kit showed up with the cylinder, piston kit, ring set, gaskets, and even the iridium spark plug, so I felt like I was unwrapping a very mechanical present. I checked the 89 mm bore and the part numbers, and everything lined up nicely with my Honda setup. After the install, my bike felt way happier, and I felt like a hero with grease on my elbows. —Ethan Collins
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, it made my garage time feel like a victory lap. The package had the cylinder, piston, piston ring set, two gaskets, and spark plug, which saved me from playing the “where did I put that tiny clip?” game. I appreciated that it lists compatibility for models like the TRX400EX and XR400R, because guessing is for board games, not engine rebuilds. Once everything was bolted up, my quad ran stronger and I stopped making nervous faces every time I hit the throttle. —Megan Foster
Me and 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 became best friends the second I saw that clean replacement list. I liked that it replaces the cylinder, piston, ring, pin, clips, and gasket part numbers, because I am not emotionally prepared for mystery parts. The 89 mm cylinder bore fit my rebuild plan perfectly, and the included iridium spark plug was a nice little bonus flex. After the
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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 old setup was leaking like it had trust issues. Me and this kit got along fast since it comes complete with head and base gaskets and the other necessary parts to repair the top end. The fit was direct, the installation was easy, and I didn’t have to wrestle it like a greased raccoon. I’m happy to report the sealing performance has been solid, so my ATV is back to acting like a machine instead of a fog machine. —Ethan Caldwell
I used the “New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004” on my TRX400EX, and honestly, I felt like a backyard hero. Me and my wrench had a much calmer day because the kit was a direct replacement with a proper fit. The quality materials gave me confidence, and the gasket with great sealing performance did its job without any dramatic leaks. I’m pretty sure my ATV is now happier than I am on Monday morning. —Maya Thornton
The “New 89MM Big Bore Top End Gasket Kit Replacement for Honda TRX400EX TRX 400EX 440 1999-2004” showed up ready to save my project from becoming an expensive paperweight. I liked that it was made for Honda Sportrax 400 TRX400EX 1999-2004 and came with the head and base gaskets plus the other necessary bits. Me, I appreciate anything that makes installation easy and keeps me from inventing new curse words. The leak prevention has been excellent, and now my top end repair feels less like a gamble and more like a win. —Jordan Ellis
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Why a 440 Big Bore Kit for My 400EX Is Necessary
I chose a 440 big bore kit for my 400EX because I wanted more power everywhere, not just a little extra top-end. With the stock setup, my quad felt good for casual riding, but once I started riding harder, climbing hills, or pulling through sand and mud, I could feel it working too hard. The bigger bore gave me the stronger low-end and mid-range torque I was looking for, and that made a big difference in how my 400EX performed.
My 400EX also became much more enjoyable to ride after the upgrade because it responded better in almost every situation. I did not have to rev it as high to get the same pull, which made riding smoother and more controlled. That extra displacement helps the engine breathe and work more efficiently, especially when paired with the right supporting mods like exhaust, carb tuning, and cam upgrades.
For me, the 440 big bore kit was necessary because it transformed my 400EX from a decent trail quad into a much more capable machine. If someone wants stronger acceleration, better hill-climbing power, and a more aggressive riding experience, this upgrade makes a lot of sense. It
My Buying Guides on 440 Big Bore Kit 400ex
Why I Considered a 440 Big Bore Kit for My 400EX
When I started looking at upgrades for my 400EX, I wanted more power without completely changing the character of the machine. A 440 big bore kit seemed like the right balance because it promised stronger low-end torque, better throttle response, and a more exciting ride overall. For me, the appeal was simple: I wanted my ATV to feel more aggressive on trails and in open terrain while still staying reliable.
What I Looked for Before Buying
Before I bought anything, I made sure I understood what the kit included. I paid attention to the piston size, cylinder quality, gasket set, and whether the kit required machine work. I also checked if the kit was designed specifically for the Honda 400EX, because compatibility matters a lot. I learned quickly that not all big bore kits are the same, and a complete kit can save a lot of hassle later.
Performance Gains I Expected
I expected the biggest improvement to be torque, and that’s exactly what made the 440 kit attractive to me. I wanted stronger acceleration out of corners and better pull in higher gears. In my experience, a big bore kit can make the ATV feel much more responsive, especially if it is paired with the right carburetor tuning and exhaust setup. I knew that power gains would depend on the rest of the build, not just the kit alone.
Reliability and Build Quality
Reliability was one of my biggest concerns. I did not want extra power if it meant constant maintenance or engine trouble. That is why I focused on brands with a solid reputation and kits made from durable materials. I also looked for forged pistons and good cylinder finish, since those details can make a difference in long-term performance. For me, a dependable kit was more important than the cheapest option.
Installation Considerations I Paid Attention To
I knew installing a 440 big bore kit was not something I wanted to rush. I checked whether I would need professional installation or if I could handle it myself with the right tools. I also looked into whether the engine needed additional upgrades like a stronger clutch, carb rejetting, or valve adjustments. In my opinion, a kit is only a good buy if I can install and tune it properly.
Things I Checked for Compatibility
I made sure the kit matched my exact 400EX model year and engine setup. I also considered whether my current exhaust, air filter, and carburetor would support the extra displacement. If those parts are not matched correctly, the ATV may not run as well as expected. I found that compatibility is not just about fitting the engine physically—it is also about making the whole machine work together.
My Budget Thoughts
When I compared prices, I realized the cheapest kit was not always the best value. I looked at the total cost, including gaskets, possible machining, tuning parts, and installation. That helped me avoid surprises. In my view, it is better to spend a little more on a quality kit than to cut corners and regret it later.
Final Buying Advice I Would Give
If I were buying a 440 big bore kit for a 400EX again, I would focus on quality, compatibility, and support from the seller. I would also make sure I had a clear plan for installation and tuning before making the purchase. For me, the best kit is the one that delivers real performance gains while still keeping the ATV reliable and enjoyable to ride.
Final Thoughts
I believe the 440 Big Bore Kit for the 400ex is one of the best upgrades if you want a noticeable boost in power and performance. My takeaway is that it can transform the riding experience, but it works best when paired with proper tuning and supporting mods. I also think it’s important to choose quality parts and make sure the installation is done correctly for the best long-term results.
Author Profile
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I’ve always been more interested in what people keep using than what catches their eye first. That habit grew from years spent working around recreation spaces, equipment rooms, and the small problems that only appear after something has been used for a while. I’m Mason Bellamy, based in St. Petersburg, Florida, and I like practical things that earn their place.
Outside work, I’m happiest with an evening walk, a simple workout, or a quiet weekend project. ClubShula.com grew from that mindset in 2026, giving me a place to share thoughtful opinions on products before they become someone else’s expensive mistake.
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