Installing a 440 big bore kit 400ex is basically the rite of passage for anyone who thinks their Honda TRX400EX is getting a little too comfortable in the slow lane. Let's be real for a second—the 400EX is a legend. It's arguably one of the most reliable, easy-to-ride sport quads ever built. But, if you've been riding one for a while, you know the feeling of watching your buddies on their fuel-injected 450s disappear into a cloud of dust the second the trail opens up. You love the couch-like comfort and the reverse gear, but you're itching for that extra grunt.
That's where the 440 kit comes into play. It's the go-to modification for guys who want to keep their air-cooled soul but need enough power to actually make things interesting when they hit the throttle. It changes the character of the bike entirely, turning it from a "beginner-friendly" trail machine into a high-torque tractor that can climb just about anything.
What Actually Comes in a 440 Kit?
If you're browsing for a 440 big bore kit 400ex online, you'll see a bunch of different packages. Most of them are pretty straightforward. At the bare minimum, you're looking at a new, larger piston (usually 89mm), a new sleeve for the cylinder, and a set of gaskets.
Some "deluxe" kits might include a whole new cylinder so you don't have to worry about sending yours off to a machine shop, while others throw in things like a high-performance camshaft or heavy-duty head studs. If you're just getting the piston and sleeve, keep in mind that you can't just "drop this in." Your stock cylinder has to be bored out to accept that massive new sleeve. It's a bit of a process, and it's not something you want to mess up with a hand drill in your shed.
The Performance Jump
So, what does it actually feel like? It's not just "a little faster." The most noticeable change is the torque. A stock 400EX is pretty "revvy" but doesn't have a ton of bottom-end punch. Once you throw a 440 big bore kit 400ex into the mix, that changes. You'll find yourself shifting less often because the motor has enough "oomph" to pull through corners in a higher gear.
It feels like the bike finally has the lungs it was meant to have. If you're a trail rider who deals with technical climbs, mud, or tight woods, this is where the 440 shines. You can lug the engine way lower without it stalling, and when you poke the throttle, it responds right now. It doesn't necessarily turn it into a 450R—those are high-revving race machines—but it makes the 400EX feel like a 400EX on steroids.
The Elephant in the Room: Heat
We have to talk about the trade-off. The 400EX is air-cooled, which is great for simplicity, but it's a bit of a headache when you start making the engine bigger. More displacement means more fire in the hole, and more fire means more heat. When you install a 440 big bore kit 400ex, you're thinning out the area around the cylinder and asking the cooling fins to do a lot more work.
If you just slap the kit in and go riding in the desert in July, you're gonna have a bad time. Most guys who do the 440 build also look into "supporting mods" to keep things from melting. An oversized oil tank or an aftermarket oil cooler (often borrowed from an XR400 dirt bike) is almost a requirement if you want the engine to last more than a season.
Reliability and the Dreaded Stud Pull
One of the most common horror stories you'll hear about the 440 big bore kit 400ex involves the head studs. Because you're increasing the bore and often the compression, the pressure inside that head is way higher than what Honda's engineers originally planned for back in the late 90s.
It's not uncommon for the stock head studs to literally pull right out of the engine cases. You'll be riding along, and suddenly you've got a massive head gasket leak because the bolts aren't holding things down anymore. If you're going to do this build, do it right. Spend the extra hundred bucks on heavy-duty head studs. Most builders swear by them. It's one of those "buy once, cry once" situations where a little extra work during the install saves you from a total engine rebuild six months later.
Don't Forget the Carburetor
You can't just give the engine a bigger stomach and not give it more food. If you run a 440 big bore kit 400ex with stock jetting, it's going to run lean, get even hotter, and probably pop. At the very least, you'll need to re-jet your stock carb.
However, a lot of people find that the stock carb becomes a bottleneck. To really see the benefits of the 440cc displacement, many riders upgrade to a 450R carb or an Edelbrock unit. This lets the engine breathe better and ensures you're getting enough fuel to keep those internal temps under control. If you stay with the stock carb, expect to spend a few afternoons swapping jets and moving the needle clip until you find that "sweet spot."
Is It a DIY Project?
Can you do this in your garage? Maybe. If you're comfortable taking the top end of a motor apart, you can definitely handle the teardown and reassembly. But like I mentioned earlier, the machine work is the tricky part. You'll need to send your cylinder to a shop that knows what they're doing. They have to press out the old sleeve, bore the cylinder, press in the new 440 sleeve, and then deck the surface so it's perfectly flat.
If you try to shortcut this or use a shop that doesn't specialize in ATVs, you might end up with a sleeve that "drops" or a head gasket that never seals. If you're not confident in your wrenching skills, this might be the time to call a buddy who has a lift and a better set of tools than you do.
The Competition: 416 vs. 426 vs. 440
Before you pull the trigger on a 440 big bore kit 400ex, you'll probably hear people talking about the 416 or 426 kits. Why would anyone go smaller? Reliability, mostly. The 416 and 426 kits don't usually require a new sleeve—you just bore out the stock one. They run a lot cooler and don't have the same reputation for pulling head studs.
But, if you're looking for the maximum power possible without swapping the whole engine, the 440 is the king. It's the biggest you can go before you have to start getting into serious "stroker" territory which involves taking the bottom end apart. If you want that "big bike" feel and you're willing to stay on top of your maintenance and cooling, the 440 is the way to go.
Final Thoughts on the Build
At the end of the day, a 440 big bore kit 400ex is about making a great bike even better. It's about not having to downshift three times when you hit a hill. It's about that satisfying "thump" of a big-bore four-stroke that sounds like a tractor and pulls like a freight train.
Sure, it takes a bit more work than a simple bolt-on exhaust, and yeah, you have to be more careful about heat and maintenance. But when you're out on the trail and you realize you're keeping up with bikes that cost three times as much as yours, it all feels worth it. Just don't forget the heavy-duty studs, keep an eye on your oil, and enjoy the ride. Your 400EX has a lot more life in it than you think; sometimes it just needs a little extra displacement to show it.