Yamaha KYB Motocross Fork Technical Reference

This reference tracks the major Yamaha KYB motocross fork generations,

with emphasis on cartridge architecture, fork size, cartridge rod size,

and major changes relevant to suspension service and tuning.

Important terminology:

“Open cartridge” and “closed cartridge” describe the cartridge architecture.

“AOSS/AOS” describes Yamaha’s Air/Oil Separation System.

“SSS” describes Yamaha’s Speed Sensitive System damping philosophy.

SSS is therefore not simply another name for a closed cartridge.

Yamaha YZ Two-Stroke Forks

Years	Model	Fork	Cartridge	KYB System	Cartridge Rod	Spring	Major Technical Notes
1991–1992	YZ125 / YZ250	43 mm	Open	KYB conventional	16 mm*	Coil	Early 43 mm KYB inverted fork. Conventional open-cartridge architecture.
1993–1995	YZ125 / YZ250	43 mm	Open	KYB conventional	16 mm*	Coil	Revised 43 mm generation. Open cartridge remains the basic architecture.
1996–2000	YZ125 / YZ250	46 mm	Open	KYB conventional	16 mm*	Coil	46 mm KYB open-cartridge generation.
2001–2003	YZ125 / YZ250	46 mm	Open	KYB conventional	16 mm*	Coil	Later 46 mm open-cartridge generation with model-specific damping revisions.
2004	YZ125 / YZ250	48 mm	Open	KYB Open Cartridge	16 mm*	Coil	Final 48 mm open-cartridge YZ125/YZ250 generation. This is the last major two-stroke YZ open-cartridge platform before the AOSS closed cartridge.
2005	YZ125 / YZ250	48 mm	Closed	AOSS / AOS	16 mm*	Coil	Major cartridge architecture change. Air and oil are separated between the outer fork chamber and the damping cartridge. First production Yamaha YZ twin-chamber generation.
2006–2007	YZ125 / YZ250	48 mm	Closed	SSS	18 mm cartridge rod*	Coil	Introduction of Yamaha SSS. Major change in damping philosophy toward speed-sensitive damping. Cartridge architecture remains closed/twin chamber.
2008–2018	YZ125 / YZ250	48 mm	Closed	SSS	18 mm cartridge rod*	Coil	Mature SSS platform. Cartridge length and internal components differ from later YZ-F applications. Model-specific piston, shim stack and damping specifications.
2019–2022	YZ125 / YZ250	48 mm	Closed	SSS	Small-rod SSS generation*	Coil	Yamaha retained an older/smaller-cartridge-rod version rather than adopting the latest YZ-F cartridge configuration.
2023–2026	YZ125 / YZ250	48 mm	Closed	SSS	Model-specific	Coil	Continued SSS twin-chamber platform. Do not assume internal interchangeability with earlier YZ125/YZ250 cartridges.
2016–2026	YZ250X	48 mm	Closed	SSS	Model-specific	Coil	Enduro version of the SSS fork with model-specific valving, spring and cartridge specifications.

Yamaha YZ-F Four-Stroke Forks

Years	Model	Fork	Cartridge	KYB System	Cartridge Rod	Spring	Major Technical Notes
2001–2003	YZ250F	46 mm	Open	KYB conventional	Model-specific	Coil	First-generation YZ250F KYB open-cartridge fork.
2003–2004	YZ450F	48 mm	Open	KYB conventional	Model-specific	Coil	48 mm KYB open-cartridge generation.
2005	YZ250F / YZ450F	48 mm	Closed	AOSS / AOS	Model-specific	Coil	First Yamaha YZ-F twin-chamber cartridge. Air and oil separation isolates the damping cartridge from the outer fork chamber.
2006–2007	YZ250F / YZ450F	48 mm	Closed	SSS	24 mm interface*	Coil	SSS introduced. Yamaha changed the damping system substantially while retaining the closed/twin-chamber architecture.
2008–2009	YZ250F / YZ450F	48 mm	Closed	SSS	24 mm*	Coil	Shorter KYB cartridge family. Cartridge length differs from later YZ-F applications.
2010–2013	YZ250F / YZ450F	48 mm	Closed	SSS	24 mm*	Coil	Long/short cartridge transition varies by model year. Verify cartridge length before interchange.
2014–2017	YZ250F / YZ450F	48 mm	Closed	SSS	24 mm*	Coil	Long KYB SSS cartridge family. Model-specific piston and valving specifications.
2018	YZ250F / YZ450F	48 mm	Closed	SSS	25 mm*	Coil	Important cartridge generation change. Later YZ-F models moved to a larger cartridge-rod interface.
2019–2022	YZ250F / YZ450F	48 mm	Closed	SSS	25 mm*	Coil	Modern YZ-F SSS cartridge generation. The 2019 YZ250F uses a distinct KYB internal cartridge assembly.
2023–2024	YZ250F / YZ450F	48 mm	Closed	SSS	25 mm*	Coil	Continued evolution of the SSS system with model-specific damping and chassis specifications.
2025–2026	YZ250F / YZ450F	48 mm	Closed	SSS	25 mm*	Coil	Current-generation KYB SSS spring fork. Internal specifications and stock valving are model/year specific.

Yamaha YZ-FX / Cross-Country KYB Forks

Years	Model	Fork	Cartridge	System	Spring	Technical Notes
2015–2018	YZ250FX	48 mm	Closed	SSS	Coil	Cross-country SSS fork based on the Yamaha KYB twin-chamber platform with FX-specific settings.
2016–2018	YZ450FX	48 mm	Closed	SSS	Coil	FX-specific spring and damping specifications.
2019–2026	YZ250FX / YZ450FX	48 mm	Closed	SSS	Coil	Later-generation KYB SSS cartridge system. Do not assume MX YZ-F valving or cartridge components are interchangeable.

Years

Model

Fork

Cartridge

System

Spring

Technical Notes

2015–2018

YZ250FX

48 mm

Closed

SSS

Coil

Cross-country SSS fork based on the Yamaha KYB twin-chamber

platform with FX-specific settings.

2016–2018

YZ450FX

48 mm

Closed

SSS

Coil

FX-specific spring and damping specifications.

2019–2026

YZ250FX / YZ450FX

48 mm

Closed

SSS

Coil

Later-generation KYB SSS cartridge system. Do not assume MX

YZ-F valving or cartridge components are interchangeable.

KYB Cartridge Architecture

System	Architecture	Main Oil Chamber	Damping Cartridge	Primary Characteristic
Open Cartridge	Open	Shared with cartridge	Open to fork oil	Traditional cartridge design. Oil level strongly influences the air-spring effect.
AOS / AOSS	Closed / Twin Chamber	Separate	Sealed cartridge	Air/oil separation. Damping cartridge operates in its own controlled oil environment.
SSS	Closed / Twin Chamber	Separate	Sealed cartridge	Speed-sensitive damping philosophy. Damping response is strongly influenced by piston velocity.
PSF	Closed / Air-sprung	Separate	Sealed cartridge	Pneumatic spring replaces the conventional coil spring. Used on selected KYB-equipped motorcycles rather than the standard YZ spring-fork platform.

KYB Fork Identification Guide

Identification Feature	What It Tells You
43 mm inner tube	Early YZ125/YZ250 KYB generation. Generally open cartridge.
46 mm inner tube	Mid-1990s through early-2000s YZ125/YZ250 open-cartridge generation.
48 mm + open cartridge	2004 YZ125/YZ250 is the important reference year. Four-stroke applications also had 48 mm open KYB forks before the 2005 twin-chamber transition.
48 mm + twin chamber + 2005	AOSS/AOS generation.
48 mm + twin chamber + 2006+	SSS family, although exact internal configuration varies considerably by model and year.
Cartridge rod/interface diameter	Extremely useful for identifying cartridge generation and determining compatibility of service tools, seals, preload spacers and aftermarket cartridge components.
Cartridge length	Critical when swapping KYB cartridges between motorcycles. Yamaha has used both shorter and longer SSS cartridge configurations.