This is from memory, because I did the job like a year ago and my book is at home, but I have a vague recollection of going through the same frustration. I unscrewed the "cap" (for lack of a better word) with a 14mm hex, and then tried to pull the tensioner out and couldn't until I reread the manual and grasped that what you are describing as a "spline" was a star shaped head that receives a 17mm hex driver and actually unscrews the tensioner itself. Got it? The star shaped part you're looking at under the cap doesn't *pull* out, it *unscrews*, with a 17mm hex wrench.