I know this question is old, but I found this site looking for the answer to this question myself. After completely breaking the exhaust cam, and sprocket in my spare engine, and trying a press and many other options, I found out that what appears to be a splined shaft inside the sprocket, is actually the head of a bolt. If you get the correct socket for it and then remove it the sprocket will pop out the end of the head allowing access to the little bolts hidden behind it. | 
