I was given this older Criterion boring head and I think it's just too large for my Rockwell 21-100 mill. It says Criterion Beverly Hills on the side of it. I don't have it in front of me to measure but I think it holds a 1" tool shank. I've not used a boring head before but could see needing one down the road, a smaller one however like the 202-8024 kit from Shars. Here's a picture of it, curious to see what you all think. -John