#0721-0005 - Standard Centering Head Curv-O-Mark Centering Heads aid in the setting of centerlines, establishing angles and marks for butt-ins, locating points inside pipes and tanks, laying out keyways and measuring declivity. The Standard Model (shown above) is used on pipe 1/2-in. diameter and larger. It has a 4-in. Y-type head and is fitted with an adjustable dial bubble protractor (DSL) and a manually operated hardened centering pin.