Robert,
When tapping a thin-walled spar I use fine threaded fasteners and taps. I then use blue Loc-Tite or Tefl-Gel on the threads.
A good US made tap & die set is VERY, VERY expensive these days. Probably easiest to figure the drill, tap & dies you will use most and build your own set..
I use almost exclusively drills, taps & dies made by Norseman/Viking. Norseman/Viking are the SAME company but sell under both names I suspect to get more market and dealers. These are premium quality USA made tools, but not inexpensive....
I get my Norseman/Viking tools from KL Jack here in Maine......
Norseman Drill Co.
http://www.norsemandrill.com/
Viking Drill Co.
http://vikingdrill.com/index.html
As you can see one in the same....
One of my favorite tools for the boat are the Norseman/Viking 29 piece round drill index sets. They are in a round plastic container with screw on lid and an EXCELLENT gasket seal. The set is totally waterproof and stays closed in a tool bag, unlike metal indexes. I use this set for 85-90% of the drilling jobs I need to do on a boat. These drills also eat SS for lunch... Both Norseman and Viking offer the same sets... I think at last count I had three of these indexes. I like them because KL Jack stocks the replacement drills from Norseman/Viking and when I break one I can easily replace it with a matched drill..
http://www.amazon.com/Norseman-29pc-16- ... m_sbs_hi_3