Honda TRX400EX / XR400R 440cc Big Bore Kit - 89mm 10:1 Piston + Complete Gasket Kit (1996-2016)

Breathe New Life Into Your Honda — 440cc of Reliable, Affordable Power

Stop leaving power on the table with a stock 400cc cylinder. This complete 440cc big bore kit from 50 Caliber Racing bolts directly onto all Honda TRX400EX, TRX400X, and XR400R models from 1996–2016. The 89mm cylinder features an improved design with a larger sleeve for longer-lasting performance, paired with a hard anodized 10:1 piston dome for improved compression and throttle response. Everything you need is in the box — no hunting for separate parts. If you can turn a wrench, you can install this kit.

Why Honda Owners Choose This Big Bore Kit

• True Bolt-On Install — No machining or modifications required — installs with basic mechanical skills
• 89mm 10:1 Piston — Hard anodized dome for improved compression and heat resistance
• Improved Cylinder Design — Larger sleeve for longer-lasting performance vs. stock
• Complete Kit — Cylinder, piston, rings, gaskets, wrist pin, circlips, and O-rings all included
• Fits 20 Years of Honda Models — Compatible with TRX400EX, TRX400X, and XR400R from 1996–2016
• Affordable Power — Significant displacement gain at a fraction of the cost of a new engine

Perfect For

🏍️ Trail and recreational riding  |  🏜️ Desert and dune riding  |  🏎️ Track and racing builds  |  🔧 ATV restoration and refresh  |  💪 Performance upgrades on a budget

Complete Kit Includes

• ✓ 89mm cylinder with improved sleeve design
• ✓ 10:1 piston with hard anodized dome
• ✓ Piston ring kit
• ✓ Complete gasket kit
• ✓ Wrist pin
• ✓ Circlips
• ✓ O-rings

Technical Specifications

Vehicle Fitment

• Honda TRX400EX (1996–2016)
• Honda TRX400X (1996–2016)
• Honda XR400R (1996–2003)

Why a Big Bore Kit Is the Best Bang-for-Buck Honda Power Upgrade

A big bore kit is the most cost-effective way to add displacement to a four-stroke engine. Going from 400cc to 440cc is a 10% displacement increase — which translates directly to more torque across the entire RPM range, not just at peak power. You feel it in throttle response off the bottom, in mid-range pull on climbs, and in top-end confidence at speed. The 10:1 compression ratio on this piston is aggressive enough to wake up the engine without requiring race fuel — it runs on pump gas. The hard anodized dome resists heat and wear better than a standard piston, which means this kit lasts. If your Honda is tired or you just want more, this is where to start.

Installed this on my TRX400EX in an afternoon. The difference in throttle response is immediately noticeable. Kit had everything I needed and the instructions were clear. — Verified Buyer

My XR400R had a tired top end. This kit brought it back to life and then some. Bolt-on as advertised — no machining, no surprises. — Verified Buyer

100% Risk-Free With R1 Protect

• 30-Day Returns — Not the right fit? Send it back, no hassle
• Fast Shipping — In stock and ready to ship
• Expert Support — 801-899-2520 | Real UTV techs, not a call center

Stop riding a tired 400. 440cc bolt-on kit, complete with everything — more power, same reliability.