Please double check your model number, as the one you provided is missing a digit.
If your model number is 316.791910, you'll find parts, diagrams, and your owner's manual on Sears PartsDirect here: searspartsdirect.com/model/fq4qpp9afp-000247/craftsman-316791910-gas-line-trimmer-parts
You'll find directions for removing the trimmer head on page 9 of your owner's manual, and the Drive shaft/handle/shield diagram shows all of the parts in the trimmer head.
If you have a different model number, enter your model number on www.SearsPartsDirect.com
for the information that pertains to your model. For help with parts, you can call Sears PartsDirect at 1-800-366-PART.
Hope this helps!