I have a stash of 3/8" shank Forstner bits from several different brands including Porter Cable, Steelx and a few others. None of them match this extension from the pack. It works, but it was just unusable. I was bored out a bit more to make it work for my project, which wasn't an easy task as this tool is made of hardened steel. After modification it worked as intended Diameter that fits this holder but I don't have one. The smallest shank in my collection measured with a micrometer is .373 and it still doesn't fit. The shank of my bit was smooth with no burrs.