Addressing the different rate of thermal expansion between aluminum andsteel, Chrysler employed unique upper and lower main-bearing caps. Theupper caps (installed in the block) are specific to aluminumapplications and feature roll sleeves to assure proper positioning. Thesleeves are used on the driver side of the block only and must never beinstalled on the passenger side. A critical oil passage intersects thepassengerside main cap bolt hole to feed oil to the main bearings. Afteralign-boring, iron-block lower caps can be used with aluminum blocks.