Correct air gap is, as Husky man says, 0.3 mm or 0.012" if you're old school!
Thanks to
http://www.acresinternet.com/cscc.nsf/ed1d619968136da688256af40002b8f7/246f20f9997db6be88256b5200157ab9?OpenDocument
for the info, cut 'n' paste the link for more detail.
Doug, November 2009