Subaru blocks are made out of aluminum and can sometimes react adversely to heat, slightly changing clearances with temperature changes. This consequently makes the engine less consistent and less reliable. Inserting a Ductile Iron Dry Sleeve into an aluminum Subaru block reduces the amount of warp, and remains a consistent size and shape through its operating heat range.
Dry Sleeve means that the sleeve does not contain water/coolant passages bored through the sleeves. This solid design provides a more rigid structure to prevent warping and sleeve expansion.
These iron sleeves come as a 98.8mm bore, and can be bored out to 102mm.