Yes, it's a crack, but it's repairable. Vendors on Ebay sell threaded, tapered cast iron plugs and the tap to install them. Find both ends of the crack and drill a hole to prevent it from spreading further, then install a series of overlapping plugs to "sew" the crack. Use a sealant like green Loctite on the plugs as you install them. Cut each plug off level with the block before drilling and tapping the next hole to overlap the plugs with each other.
Jerry
