BL-123 Pair of 1" x 2" x 3" Precision Steel 1-2-3 Blocks Go to amazon.com Related posts Best 123 block metric FeaturePrecision ground. Case hardened.Cross-drilling design for extra strength and minimum weight.Five 3/8-16 tapped holes are provided for clamping. DescriptionHeat treated above 48 Rockwell "C"; .0001 to .0003 over allowance for nominal lapping. Squareness of all sides is within .0002 per inch.