The Model 300 Standard Base is the most popular model available. This base will accept all 300 series Heads and any 5/8" diameter shaft. The Model 300 creates a versatile work holding system by selecting two or three different Heads. The single control knob and exclusive "split-ball", locks the Head firmly in place. This base features a 4.3125" bolt circle, countersunk for three, 1/4" flat-head screws (not included).