For resting a part on a rounded surface or hole location 

NC = NYLON   SC = STEEL  AC = ALUMINUM

PART #    THREAD     DIA.     HT.
R20-NC   1/4"-20    .625"     .50"
R20-SC   1/4"-20    .625"     .50"
R20-AC   1/4"-20      1.0"     .75"

PART #    THREAD     DIA.     HT.
R8-NC        M8       16mm   13mm
R8-SC        M8       16mm   13mm
R8-AC        M8       25mm   25mm