cwasil and mudder are working on a chart that shows bushing diameters for for different pen kits. ODs will vary from kit to kit. There are only two standard pen mandrel sizes so IDs for bushings will be one or the other. Most people will be using the so called "7 mm" mandrels which ranges in diameter from 0.243" to 0.247" depending on the maker. The "8 mm"(B) mandrel has a diameter of 0.291" (I think) and is not used on very many kits. The "B" mandrel is made by Berea and used with a few of their kits. Rockler, Hut, Woodcraft and a few others resell some of the Berea kits and also require the larger mandrel.