phil. stainless is softer than steel, what you need is a hard grub screw from a good manufacture like UNBRAKO to name one . you can tell because there a bitch to shorten, the hacksaw screams when you cut them, same goe,s for l shaped keys, a good one [unbrako for eg] will bend and almost snap before you round out the grub screw. and the flats only have to be tiny 2mm across at most. this is a minute piece to grind from the axle. you want cup point screws and i garantee? you,ll never have a wheel come off again. slightly off subject the flat hub wheels i use, but i drill holes round the centre of the out side dia and use tyres with the widest internal step then glue the edges of the tyres to the rims using a 20-30 sec c/a glue which gives you plenty of time to get the tyres on straight voilla! air tyres that work really well. john. thi is a quick edit ive just remembered, using a steel grub srew in an ally wheel immediately puts it out of balance grinding the small 2mm wide flat on the axle near as dammit puts them back in balance, and if you strip a 2mm grub screw the hole left is the perfect size to tap them 2.5 mm. a cheap chinese tap is good enough if youre carefull 1 turn in 1/2 turn out till the hole is tapped and use lubricant.