I was doing some experimenting with P&C cotton yarn a while back, because I wanted to be able to pull a doubled piece of yarn through a micro coil for my GP Spheroid. A coil wrapped around a 5/64 drill bit was too tight. 3/32 was too loose. I needed something in between for my coil. I found that a #44 drill bit, which is .086" diameter (2.18mm) was prefect. It pulls through with just the right amount of friction. I wrap the coil on the #44 bit, bend a piece of kanthal into a V, double a piece of yarn into the V, and pull it through.
I have several attys set up this way, including a couple Kayfuns, Squapes, Taifun GTs, and an Omega dripper with dual coils, in addition to the Spheroid.