Somehow my reply to the OP has become truncated. So I will try to recreate it. The straight edges need to be long enough to span the engine block diagonally. The block must be checked lengthwise, crosswise, and diagonally. After the block has been scrupulously cleaned lay the straight edges on the block and use feeler gages to determine flatness. Just try to slip a feeler gage under the straight edge all along its length. Start with the .001" feeler and increase as needed. Put only enough pressure on the straight edge to insure it doesn't lift when trying to slip the feeler gages between the straight edge and the block. Before trying to determine flatness it is a very good idea to make sure there are no tiny high spots on the block. A tiny ding will raise material around the ding. You can use a FINE india stone with diesel or WD40 or a similar thin lubricant to check for high spots. Just slide the stone across the wet surface and if the stone hangs you have a high spot. Remove the high spot by running the stone across the high spot until the stone no longer hangs up. Use moderate pressure when doing so. If you don't have an india stone and can't find one easily then you can use a NEW fine cut file to remove high spots. It will tend to hang up just like a stone but will cut much faster. So use light pressure. Using a fine india stone and thin lubricant you can detect easily a high spot that is only .0001" above the surface being checked.
Eric
