I used to always wear a backpack and lugged my water bottles in them when I ride.  I’ve since grown out of that and am now ready to put bottle cages on my bikes.  Of course they have to be LIGHT!
 
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I bought these off eBay from a Chinese vendor. They took over a month to arrive but so far its been worth the wait.
 
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15 grams a piece!  pretty good.
 
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They came with these stainless steel bolts and washers that are horribly heavy.
 
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I decided to instead use one aluminum bolt in combination with a supporting Nylon bolt.   The weight savings is tremendous. 
 
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I mounted the Aluminum bolt on the top where I figure most of the stress from inserting/removing the bottles will be transferred to the mounting bolt.  TH bottom bolt should only be resisting a shear force from the torque on the cage so the Nylon bolt should be enough for this application. 
 
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They look pretty slick.  The problem with these “half” cages is that the bottom of the bottle tends to wiggle around a bit.  I might play with putting some sticky tape on the end of the cage if the problem proves to be big.