Yamaha YZ250 combustion chambers

Chamber selection:

1999-2022

Y20 - 22cc Stock compression for the X model 

Y21 - 21.1cc Stock (95+ octane)

Y22 - 20.1cc (100+ octane)

Y23 - 19.1cc (105+ octane)

1999-2022 big bore chambers:

YZ295 - 72mm 93 octane - Revised squish clearance .055"

1988-1998

Y30 - stock

Y31 - (95+ octane)
Y32 -  (100+ octane)
Y33 -  (105+ octane)