Vertex Exhaust Gasket Kit for Yamaha YZ250X & YZ250 Two Strokes

The Vertex Exhaust Gasket Kit (823139) is an OEM-replacement cylinder-to-pipe sealing and retention kit engineered for 2001+ Yamaha YZ250 and YZ250X two-stroke motorcycles. Two-stroke expansion chambers endure extreme exhaust gas temperatures and constant high-frequency engine vibration. Over time, thermal cycling hardens and flattens the dual rubber manifold O-rings while stretching the exhaust tension springs. Once the seal degrades, unburned two-stroke oil ("spooge") weeps onto the cylinder and front engine cases, and ambient air is drawn into the exhaust port during scavenging pulses—disrupting backpressure and leaning out jetting.

Manufactured to meet or exceed Yamaha factory specifications, the 823139 kit includes two high-temperature fluorocarbon (Viton) exhaust manifold O-rings and two heavy-duty swivel-hook tension springs. The high-temp elastomer rings resist thermal hardening and chemical breakdown from synthetic premix oils, while the high-tensile springs maintain consistent clamping tension between the expansion chamber collar and cylinder exhaust spigot across stock and aftermarket exhaust systems.

Features:

• Complete Cylinder-to-Pipe Service Kit: Includes both exhaust manifold O-rings and both tension springs needed to reseal the primary exhaust port junction.

• High-Temperature Viton O-Rings: Fluorocarbon elastomer rings withstand sustained two-stroke exhaust temperatures and synthetic premix oil exposure without flattening or cracking.

• Swivel-Hook Tension Springs: High-tensile spring steel construction with rotating swivel ends reduces harmonic vibration fatigue and prevents hook breakage.

• Eliminates Oil Leaks & Air Intrusion: Restores an airtight seal at the cylinder flange to stop black exhaust oil drips and prevent lean air leaks during exhaust scavenging.

• Direct OEM Cross-Reference: Replaces Yamaha factory exhaust seals (5MW-14642-00-00) and tension springs (90507-20052-00).

• Universal Pipe Compatibility: Fits OEM Yamaha expansion chambers as well as aftermarket two-stroke pipes (FMF Gnarly, FMF Fatty, Pro Circuit Works/Platinum, Scalvini).

What's in the Box:

• 2x High-Temperature Viton Exhaust Manifold O-Rings (Replaces OEM #5MW-14642-00-00)

• 2x Heavy-Duty Swivel-Hook Exhaust Tension Springs (Replaces OEM #90507-20052-00)

Fitment:

• 2016–2027 – Yamaha YZ250X

• 2001–2027 – Yamaha YZ 250

Fitment Notes:

• Generation Incompatibility Warning: DoesNOT fit 1999–2000 Yamaha YZ250 models (which require Moose kit0934-5322 / Vertex823140) or pre-1999 YZ250 models. DoesNOT fit YZ250F or YZ250FX four-stroke models.

• Silencer Coupler Note: Services the primary cylinder-to-expansion-chamber joint; does not include the rubber pipe-to-silencer joint sleeve (Yamaha OEM #3JE-14615-01-00).

• OEM & Aftermarket Part Number Cross-Reference:

  • Replaces Yamaha OEM Exhaust Seal #5MW-14642-00-00 (Qty 2)

  • Replaces Yamaha OEM Tension Spring #90507-20052-00 (Qty 2)

  • Cross-references Moose / Winderosa Part # 0934-5321 /823139MSE

• O-Ring Groove Cleaning & Silicone Lubrication: Before installing the new O-rings, inspect the two internal seal grooves inside your expansion chamber collar (or cylinder spigot, depending on pipe brand). Baked-on carbon and two-stroke oil residue frequently pack into the bottom of these grooves, preventing new O-rings from seating fully and causing the pipe to bind during installation. Scrape out hardened carbon with a brass pick, clean the grooves with Maxima Electrical Contact Cleaner, and seat the new O-rings cleanly. Coat the O-rings and the cylinder exhaust flange with a thin film of Permatex Ultra Copper RTV (81878) before sliding the pipe into place—installing dry O-rings can cause them to pinch, roll out of the groove, or tear.

• Swivel-Hook Spring Orientation & Safe Installation: Always install and remove exhaust springs using a dedicated T-handle spring puller tool rather than pliers or flathead screwdrivers, which can slip and possibly puncture radiator cores. Orient the rotating swivel end of each spring toward the cylinder head hook so the spring can pivot freely under high-frequency two-stroke vibration without fatiguing or snapping the hook loop.

• Stress-Free Expansion Chamber Alignment: Two-stroke pipes undergo substantial thermal expansion and frame flex on the trail. Always hook up the two cylinder flange springs and slide the stinger pipe into the rubber silencer couplerbefore tightening the two frame isolator mounting bolts. Leaving the frame bolts loose until the pipe settles naturally into its resting position prevents side-loading the new O-rings and eliminates premature cracking of pipe mounting tabs.

• Maintenance Interval: Replace both exhaust O-rings and tension springs whenever installing a new expansion chamber, performing a top-end piston service, or noticing black oil residue misting around the cylinder head flange.

Part Number: 823139