"HHIP 36 Piece Space Block Set Four 1/4""-28 connecting screws are included to make any length desired. Each block has an internal 1/4""-28 thread. Meets or exceeds federal specifications. Work surface is hardened to 57-60 RC. Made of precision tool steel. Individually marked. Complete in fitted case. Set includes 9 blocks that are .101 to .109 by .001"", 15 blocks that are .050 to .190 by .010"", 9 blocks that are .200 to 1.000 by .100"", 2 blocks that are .0625"" and .125"" and 1 extra block that is 1.000"". Accuracy: ±0.0001"""