best big bore kit for 400ex

I went with a 440 from Powroll years ago and it's been bulletproof. Paired it with a hotcam and FCR carb. Runs strong on trails and never overheats as long as you keep the jetting clean. I'd avoid the 480 unless you're willing to do frequent top-end work.

Disagree with that. I've had a 480 kit from Sparks for two seasons now and it's been fine. You just have to run a good oil cooler and maybe a bigger radiator. The torque is addictive for climbing. Jetting is critical but not rocket science.

I'm a mechanic and see a lot of 440s come in with cracked sleeves. The 440 is more forgiving but if you choose a quality brand like CT Racing or Pro-X, you'll be okay. The 480 can be a headache if you don't upgrade the cooling system. Go 440 if you want reliability.

I did a 440 with a 11:1 piston and a Web cam. It pulls hard but I had to rejet constantly for altitude changes. The power is worth it though. I'd say get a good aftermarket header too, the stock one chokes it.

Don't forget about the carb. A 440 needs more fuel. I went with a 39mm FCR and it woke the bike up. But if you're on a budget, a rejet of the stock carb works. Just be ready to fiddle with it.

I'm a beginner and just put a 416 kit in mine. It's a nice step up without breaking the bank or the motor. Not as wild as a 440 but still noticeable. Plus it's easier to keep the stock carb with a rejet. Maybe consider that if you don't want a huge project.

The 416 is a waste of money in my opinion. You'll just want more later. Save up for a 440. The 416 doesn't change the powerband enough to justify the cost. I'd rather spend that on suspension.

Suspension first, always. I did a 440 and then realized my stock shocks couldn't handle the extra speed. Now I'm rebuilding the whole thing. If I could do it over, I'd do suspension first, then big bore.

I run a 480 with a 12.5:1 piston on pump gas with octane booster. It's a blast but it vibrates like crazy. Make sure you have good engine mounts and maybe some vibration dampening. Not for everyone.

I've seen too many 480s blow head gaskets. The 440 is the sweet spot for trail riding. You get plenty of low-end grunt without the heat issues. My advice: go 440, keep the compression around 10.5:1, and run a quality gasket kit.

What about the 416? I've had mine for 5 years and it's been great for trail riding. It's not a rocket but it's reliable. I ride tight woods and it's perfect. Not everyone needs 50 hp.

The 416 is fine for casual riders. But if you're asking about the 'best' big bore, that's the 440. It gives the best balance of power and reliability. The 480 is for drag racers or dune bashers. For trails, stick with 440.

I disagree with the 440 being best. I had a 440 that was a nightmare with jetting. Switched to a 460 from GT Thunder and it's been way more consistent. The 460 uses a different bore/stroke combo that seems to run cooler. Worth looking into.

I've built dozens of 400ex motors. The 440 is the standard for a reason. But if you want to stay reliable, don't go over 11:1 compression and use a quality cam like a Stage 2. Also, upgrade your oil pump. The stock one can't keep up with the extra heat.

I'm still on a stock motor with just a pipe and jet kit. I'm scared to do a big bore because I've heard too many horror stories about overheating. Maybe I'll just do a 416 for peace of mind. Thanks for the input.

Don't be scared. Just do your research. A 440 with proper cooling and jetting is fine. I ride in 100°F weather and my temp gauge never goes above 210. Use Engine Ice coolant and a good fan. You'll be fine.

I did a 440 and it was a pain to start when hot. Had to install a decompression release. That's something to consider. The high compression makes for a hard kick. If you don't want to deal with that, stick with lower compression.

I went with a 480 and I regret it. It's too much for the frame. The bike handles like a pig now. I'm going back to a 440. Sometimes more isn't better. Think about what kind of riding you actually do.

I love my 440. It's been 4 years and no issues. I use a Dynatek CDI and a FMF exhaust. The key is to keep the air filter clean and change oil often. These motors are tough if you maintain them.

Honestly, the best big bore kit is the one that fits your riding style and budget. I say start with a 440 from a reputable company like HMF or Sparks. If you want more later, you can always stroke it. But for most people, 440 is plenty.