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Playstation 2 - MX Unleashed cheats

Select  "Options" tab in the main menu, after press R1 or L1 to cycle 
from the "Game Settings" option. Do it until you will see "Cheats" option.
Type to here the following codes:

Unlock 50cc Class: Type SQUIRRELDOG
Open Up Pro Physics: Type SWAPPIN
Unlock all bonuses: Type CLAPPEDOUT to "Character Completion" option

Bowling: WRECKINGBALL 
Expert AI: OBTGOFAST 
Unlock All Machines: MINIGAMES 
500cc Bikes: BIGDOGS 
Freestyle Tracks: BUSTBIG 
National Tracks: ECONATION 
Supercross Tracks STUPERCROSS