a 27mm wrench works for me, you may have to move the air pump out of the way to get to it, if you are passed this point and need to remove the inner retainer, i made a tool from an old mercedes lug bolt, or a 17mm allen will work, if you are beyond that point and need to get the tensioner it's self out, push the tensioner in with your finger and use the rail to push it back out. let me know what you find i am here all morning, send email if you want