if as long as it doesn't distort the chainring, pin touching arm lightly should do no harm.
perhaps as they consider the 50 a road chainring
they thought pin didn't matter.
you could always kludge a poprivet to make a pin , or drill and tap a small screw ..
NB with aluminum a file is better than a grinder
[aluminum in grinding wheels is not good]
Last edited by fietsbob; 12-30-10 at 11:25 PM.