Darton MID Sleeves are the best sleeves on the market because of their strength and fitment. Their flanged tops align against each other while still allowing coolant to flow through them. (wet sleeves)
Many production cast iron and aluminum blocks suffer from a design weakness of cylinder stability by nature of poor support at the upper deck area. The manufacturing process of "cast in sleeves" provides for economy of scale in low horsepower engines, but does not accommodate high horsepower, high boost, or larger bore sizes.