I think I use Danville Flat Waxed Nylon size "A" thread. I get it from Mike at M&D's. It's a fly thread. I tried a rod building threat many moons ago and found it was cutting the bucktail when I cinched it tight. Could have been OE though
The A is a great size for tying bucks, use several layers of buck wrapped, then thin superglue, then another layer of buck, superglue,
it lasts longer that way