http://www.powertoolspares.com/bd_model.php?model=GL630-----A&Product_Description=STRIMMER&class1=Black+%26+Decker&class2=Outdoor&class3=Hedgetrimmers%2C+Strimmers%2C+Shears&class4=Strimmers
Part number 31 on the above website, from the breakdown diagram you can see where you need to be in the strimmer to replace it. Simply put the belt over both pulleys and you'll be back in action.
Alex, July 2009
