http://www.mccullochpower.com/ddoc/MCCI/MCCI1995_AAaa/MCCI1995_AAaa_ptmt270x_.pdf
Here is a parts list for the Strimmer. The part that you want to remove (31?) is secured with a bolt inside the center. (It is usually a Torx T20/25.) You will also need to remove the clutch plate under there - Can be tricky - Might be best left to a professional - we don't bite you know ;)
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May 2012