Sleeves are tubular inserts placed into the engine block to serve as the cylinder bore. This allows complete replacement of the cylinder bore without reboring the block to a larger size. Sleeves can be either wet or dry. Wet sleeves are inserted directly into the engine's water jacket to allow for more effecient cooling of the cylinder. Dry sleeves are inserted into a cast bore in the engine block, requiring a thermal transfer between the sleeve and the engine block, and the engine block and the coolant.