Don't loosen the allen bolt. The pulley is attached to a short bracket which then bolts to the engine. Use 19mm open end wrench to loosen the bolt that holds the bracket to the engine. You won't have to loosen it very much to cause to bracket to swing down. The belt can now be removed and replaced. The real trick is applying leverage to the pulley assembly to tighten the belt while tightening the 19mm headed bolt. They make a tool for this that uses the allen bolt that holds the pulley but you should be able to apply leverage to the bracket that holds the pulley with a combination of a short prybar and 22mm open end wrench to the bracket. Mark