🔥 88mm Big Bore Top End Kit – Honda TRX400EX / TRX400X / XR400R
Complete Performance Upgrade – Bolt-On Power With No Machining Needed
Looking for real power without having to open your crankcase or machine your cylinder head? This bolt-on 88mm big bore top end kit includes everything needed to increase displacement, improve valve lift, and keep your motor reliable for the long haul.
✅ Fits all years of Honda 400EX, 400X, and XR400R
✅ No case or head machining required
✅ Includes top-quality brands: Wiseco, Hotcams, Cometic, Talon, SKF, NGK
🧱 88mm Heavy Duty Cylinder
⚙️ 88mm Forged Piston (Wiseco or Wossner based on availability)
🔩 Nitrite-Coated Rings, Chrome Wristpin & Circlips
🏁 Hotcams Stage 2 Camshaft – Boost mid-to-top end power
🛠️ SKF Cam Bearings
🧰 Cometic Big Bore Gasket Kit
🧱 MLS Big Bore Head Gasket – Multi-layer steel construction
🔒 Talon Titanium Head Stud Kit – Lifetime Warranty*
🔧 Heavy-Duty Cam Chain Conversion
🔥 NGK Spark Plug
🎁 Stickers + 1-Year Warranty
🔧 Direct Bolt-On Installation – No machining or case boring needed
💥 Displacement Increase = Real HP Gains
🧩 Built for Reliability – All parts brand new and performance-proven
🛡️ Titanium Studs = Superior Sealing & Torque Retention
⛽ Compatible with Pump or Race Gas
Year | Model |
---|---|
1999–2008 | Honda TRX400EX (Sportrax 400) |
2009–2014 | Honda TRX400X |
1996–2004 | Honda XR400R |
✔️ 1-Year Warranty on the complete kit (new and used parts)
🚫 This is not a DIY project — professional installation is required for all warranty claims
🛠️ Warranty covers parts only after they are returned and inspected for manufacturer defects
📦 Shipping Time:
Ships within 3–5 business days. Tracking provided after fulfillment.
💬 Customer Support:
Available Monday–Friday, 7 AM to 4 PM PST
Live store chat available 24/7
We ship next day most of the time. Modquad parts are shipped Monday-Thursday. Engine Kits are 3-5 business days.
We don't do backorder! If your items has already sold you will be notified immediately and you money refunded on the spot.
Sold items are removed daily.
If you see it on this website, it is 100% in stock unless it just sold before you clicked buy.
If you call we answer, if you text we respond, if you have a question we respond. Sometimes we sleep so it could be next morning but we will respond.
Thank you for shopping with us! We appreciate you!
426 kit really rocks !!! So glad that I went this way, went to 14/36 sprockets and the R that I had wouldn't be able to handle it !!
The studs suck before I got to 33 lbs ft they rounded
Great product highly recommend it
Broke a valve on my 400ex 3 weeks before a riding trip. Ordered the kit and they shipped it fast and got it done in time. Thank you!
It's a very nice kit I haven't finished the install yet as I had to buy a clutch cover gasket and it's not top priority but hopefully this weekend Ill ride it..
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My 400ex has never sounded better. New tensioner, guides, and chain fixed the noise. Metal bottom end gasket was cool too, didn't rip while working on it.
Thank you for your positive review! We're glad to hear that the TRX400EX Cam Chain Tensioner & Wiseco Heavy Duty Cam Chain Kit was able to fix the ticking noise on your 400ex. We agree, these small maintenance steps can make a big difference in the overall performance and sound of your vehicle. We appreciate your feedback and hope to continue providing quality products for your ATV. Happy riding!
Well my mechanic said it was some good parts in the box. I'll let you know how the rebuild goes. Great I hope
I haven’t received them yet
Trevor, thank you for your 5 star review. We appreciate that. Rest assured I just checked your tracking info and UPS says they will arrive today between 1:30 & 3:00 pm. Your going to love the Lectron Carburetors!
Great quality and fast shipping thanks
I received the wrong center crankcase gasket
Hi there, thank you for bringing this to our attention. We apologize for the inconvenience of receiving the wrong gasket. We take pride in our products and want to ensure that our customers receive the correct items. We sell countless TRX400EX engine kits and we always ship the same gasket every time. In situations like this, we encourage our customers to reach out to us so we can rectify the issue. We would have been more than happy to ship out the correct gasket right away if you had sent us a picture of the wrong one. Please don't hesitate to contact us for any further assistance. Thank you for choosing our product.