Tom,
Instead of using the orginal 13mm ATF(across the flats) nut I used a 12mm
ATF . This is the copper plated exhaust nut used on aircooled engines but I
use them anywhere I can.  I buy them by the box of 100
http://www.busdepot.com/details.jsp?partnumber=059129601
The smaller nut means you can use a smaller socet.
Ken Lewis
http://neksiwel.20m.com/