enter your engine number into the search it should look a bit like this 10A900 and is stammped on the metal castings of the engine. There is a video on the site to show where the numbers can be found.
http://www.briggsandstratton.com/maint_repair/manual_and_more/doclist.aspx?category=Engines&manual_type=Operator%20Manuals&menu=nav3&subMenu=3
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February 2008