Forgive my ignorance but when I started installing Future fin boxes, they already had the one pass bits. I have a few application where I can use the reversable donut with the jig to route out broken boxes and install new ones. I need to purchase the correct bit. Does anyone know the size of the original router bit? I know the shank is 1/4" and looking at pictures, it looks like 1/2" but I need to need to know the lenght. Thanks inadvance. D