We use two sizes (OD). Shaft A (approx. 0.25" OD) and Shaft B (approx. 0.33" OD). Both shafts are straight, have a tightening nut on the tail stock end as well as a precision center drill hole to receive stop collar bushing on the headstock end. We use a large OD mandrel (Shaft B) on some of the larger kits because it provides more support and prevents "whip".