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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.
What is a big bore kit and how does it work?
A big bore kit increases your engine's displacement by using a larger cylinder and piston. This allows for more air and fuel to enter the combustion chamber, resulting in more power and torque.
Do I need to machine my engine to install this kit?
No, this kit is designed as a direct bolt-on upgrade and requires no machining. It installs using basic mechanical skills.
Will this kit work with my specific Honda model and year?
This kit is compatible with all Honda TRX400EX, TRX400X, and XR400R models from 1996 to 2016.
Can I use regular pump gas with this kit?
Yes, the 10:1 compression ratio is optimized to run on standard pump gas without detonation.
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.
— John D., California
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.
— Mark T., Texas
In Stock Now - Join thousands of Honda owners who trust R1 Industries for reliable, affordable performance upgrades.
Stop riding a tired 400. 440cc bolt-on kit, complete with everything — more power, same reliability.