I am not certain about the differences beween the R121 and R171. I buy my xfire parts from FCP (Foriegn Car Parts.com) in the US. They donot have any listings for our xfires but they will respond to models like SLK320 R121 etc. I have purchased engine & auto transmission parts from them for my 2005 xfire roadster limited which has the 3.2L engine.