Hi Michele,
I am also using a M13x0.8 tap for the cap. For the few I have made so far, the largest bit I use to drill the cap out is 37/64" which is 14.72mm. The smallest cap diameter at the threads has been 15.65mm. This leaves 0.93mm total, so 0.465mm thickness of wood around the sleeve at the thinnest point. As I think I shared in another post, I am step drilling my cap blank in 3 steps so that I have some room to taper the cap.
For the barrel, I am using a M10x1tap. The largest bit used here is 15/32" which is 11.97mm and the smallest barrel diameter at the threads has been 14.36mm. This leaves 2.39mm total, so 1.195mm thickness of wood around the sleeve at the thinnest point, not nearly as thin as on the cap. Like my caps, I step drill the barrel, but only 2 steps (smaller for the part of the converter that turns) which again allows me some wiggle room to taper them.
The above are for the smallest of the few I have made. The first of them was the narrowest and I have been leaving a little more meat on the bones as I like the look, but still, the tolerances are very tight here, I can see how easy it would be to go right through.
David