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Aluminium as they are more forgiving for errors in fitting than steel. They need less " pull up" force to set them correctly. Make sure when you are pulling them up that the tool is perpendicular to the frame. Use anti seize when fitting the cage retaining screws.
The correct drilling size for M5 nutserts is 7.1mm, try to get as close to this as possible, a little smaller is better than larger.
Very cheap on ebay aluminium M5 open ended packs of 10.
http://www.ebay.co.uk/itm/RIVNUTS-NUTSERTS-RIVET-NUTS-STEEL-ALUMINIUM-OR-STAINLESS-M3-M4-M5-M6-M8-M10-M12-/361388153801?var=&hash=item5424699bc9:g:zE4AAOSwT6pVhcB9