It is legal to run, but its not a Mugen. Its a Caster which is based on the MBX5. Roar rules for 4wd used to be that it had to be a factory available kit, but a few people petitioned it. Personally I dont agree that they should be legal. Last year in our state series the rules clearly stated "No 17mm hubs allowed on vehicles". But since 3 of the members on our series committe are sponsored Mugen drivers, that rule went away once the MSC came out.