FMF Gnarly 2 Stroke Exhaust for the 2026+ Rieju MR 300i

The FMF Gnarly 2-Stroke Exhaust Pipe (025321) is a heavy-duty, torque-focused expansion chamber engineered specifically for the 2026–2027 Rieju MR 300i fuel-injected enduro platform. Technical woods riding, steep boulder climbs, and hard enduro terrain demand immediate low-RPM throttle response and high impact resistance. Standard factory expansion chambers are constructed from thinner-gauge steel that dents easily against rocks and logs, altering internal exhaust wave tuning and choking engine output.

Constructed in the USA from rugged 18-gauge carbon steel and finished in FMF’s signature nickel plating, the 025321 Gnarly pipe is built to withstand direct trail impacts and harsh weather exposure.Its internal cone geometry is tuned specifically for Rieju’s 298cc Throttle-Body Fuel Injection (TFI) two-stroke engine, shifting exhaust scavenging efficiency downward to produce stronger bottom-end pull and a thicker midrange power curve.This broader low-end torque delivery reduces clutch abuse in technical singletrack while maintaining clean transitions across the engine's three selectable power maps.

Features:

• Heavy-Duty 18-Gauge Steel Construction:Thicker steel wall profile provides superior dent and crush resistance against boulders, logs, and trail debris compared to standard OEM pipes.

• Low-to-Midrange Torque Tuning:Expansion chamber volume and header taper are engineered to maximize bottom-end tractability and midrange pull for technical enduro riding.

• Durable Nickel-Plated Finish: Corrosion-resistant nickel coating protects the raw steel body from moisture, mud staining, and thermal oxidation.

• TFI Engine-Specific Geometry:Developed specifically for the port timing and dual-injector TFI delivery of the 2026+ Rieju 298cc two-stroke powerplant.

• OEM & Aftermarket Silencer Compatibility: Mates cleanly with the factory Rieju OXA silencer as well as FMF Powercore 2.1 and Turbinecore 2.1 silencers.

• Manifold O-Rings Included: Ships with fresh high-temperature exhaust manifold O-rings for an airtight seal at the cylinder spigot.

• Made in the USA: Designed, stamped, and hand-welded at FMF Racing in Rancho Dominguez, California.

What's in the Box:

• 1x FMF Gnarly Nickel-Plated Expansion Chamber Pipe (Part #025321)

• 2x High-Temperature Exhaust Manifold O-Rings

• 1x FMF Racing Decal Pack

Fitment:

• 2026–2027 – Rieju MR 300i

Fitment Notes:

• Generation Incompatibility Warning: DoesNOT fit 2020–2025 carbureted Rieju MR 300, MR Pro 300, MR Racing 300, or MR Ranger 300 models (those earlier models utilize the previous-generation carbureted engine and require FMF part number025218).

• Silencer Compatibility: Direct fit with the stock OEM silencer and FMF MR 300i slip-on silencers (such as FMF Powercore 2.1 #025322).

• Manifold O-Ring Lubrication & High-Temp RTV Sealing: Install the two supplied exhaust O-rings into the grooves of the FMF header spigot prior to mounting. Apply a thin film of high-temperature copper silicone such as Permatex Ultra Copper RTV over the O-rings and inside the cylinder exhaust flange. The silicone acts as an assembly lubricant so the O-rings do not pinch or roll when sliding the pipe into the cylinder, while curing into a flexible thermal seal that prevents unburned two-stroke oil ("spooge") from leaking onto the front engine cases and electronic power valve housing.

• Stress-Free Alignment Before Tightening Mounts: Two-stroke expansion chambers undergo extreme thermal expansion cycles and chassis flex. Always hook up the cylinder exhaust springs and slide the mid-pipe into the silencer couplerbefore tightening the frame mounting bolts. Leave the two chassis mounting bolts loose until the pipe settles naturally into its resting position. If a bracket tab does not align cleanly with the rubber isolator mount, loosen the isolator nut on the frame to adjust its angle rather than prying the pipe into place. Bolting down a pre-stressed pipe transfers harmonic vibration directly to the cylinder flange and increases the risk of cracked mounting tabs during trail impacts.

• TFI Fueling & Power Map Pairing: The 2026+ Rieju MR 300i utilizes a dual-injector Throttle-Body Fuel Injection (TFI) system with three handlebar-selectable maps (Race, Forest, and Wet). The FMF Gnarly 025321 is engineered to operate safely within the factory TFI fuel trims without requiring an aftermarket ECU tuner. For technical rock gardens and slick root climbs, pairing the Gnarly’s increased bottom-end torque with theForest orWet maps produces linear, tractable power delivery that minimizes rear-wheel spin.

• Post-Wash Nickel Finish Maintenance: While FMF’s nickel plating resists rust far better than bare raw "works" pipes, mud baked onto a hot header will eventually stain the finish if left sitting. After pressure-washing the bike, blow standing water off the expansion chamber with compressed air and wipe the pipe down with a light coat of Maxima SC1 or WD-40. When starting the bike after applying protective oil, wipe off excess oil from the header cone first so it does not smoke or burn dark brown spots onto the nickel plating.

Part Number:  025321