this may end up a double post but here goes. Its probably a 12-20 thread. I've posted a list of some common threads Stanley and some others used here, http://lumberjocks.com/donwilwol/blog/24090 They are not common but a tap and die set can be purchased here, http://stjamesbaytoolco.com/ just look for the 12/20 Tap And Die Set $35.00