Just because they’re deep under your bike people forget about them.  The Bolts that hold your cable guides are horribly overkill.  For this job a nylon bolt will do just fine.  If you’re extra careful, a nylon bolt with some glue to stick the guide to the frame itself will be extremely efficient.
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Around here its tough to find metric nylon bolts but a 8-32 standard bolt is pretty much the equivalent and will suffice. Ace hardware and True Value hardware stores should carry the 8-32 bolts.  The single steel M5 bolt weighs 2.9 grams.  4 nylon 8-32 bolts weigh 1.3 grams making them only .325 grams each! 
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I had plans of filing doing the big head to save some more weight but they fit the guide perfectly the way they are.
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I decided to put them on all of my bikes.  Again, they fit perfectly into the guides.
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They also work well as caps for unused bottle cage bolts.
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