Be sure existing lid diameters are smaller than "AA" dimension as shown in table. "AA" dimension in table is opening size in extension ring. Your lid should measure 1/4" to 3/8" less in diameter than this dimension.
For raising manhole covers when pavement is resurfaced.
Adjusting rings or extension rings, as they are sometime called, eliminate the necesssity of raising or replacing existing manhole frames on resurfacing projects. The cover recess is the same depth and diameter as the existing manhole frame. Amount of rise coincides with thickness of resurfacing material, thereby bringing the manhole lid to new position flush with refinished roadway surfaces.
Manhole adjusting rings can be placed rapidly and are a permanent installation. Use of adjusting rings may reduce raising costs to as little as 20% of digging up the old manhole frame and resetting it to new grade.