The carefully engineered cast iron lid and ring includes machined surfaces and bolting features reducing the danger of a lid "pop-out" in high traffic areas. Bolt down recommended for high traffic areas. The largest throat diameter for any box in its size range permits quick, easy valve adjustments.  The large throat also makes this unit highly adaptable as a survey monument box.  Unique locking grade rings assure permanence and quality of surface grades when streets are re-paved.  Telescoping extensions facilitate installation at jobsite and act to isolate pipeline from road shock.