New block with rough 3.98 diameter bores, .875 lifter diameter, 3.00 mains. Not Siamesed bore This block was originally purchased for use by a Nascar team years ago and never used. It has very heavy duty 4 bolt main webs and thick pain rails. These blocks are getting hard to find. If you plan on building a high horsepower small block this is a great base to start with. Blocks like this have been used with great success on boosted applications.