The 4.8L block was made of cast iron and carried a 12551358 casting number. This block was used from ’99-’04 for all the 4.8L motors.1999 -’04 – 5.3L TrucksThe 5.3L engines had a longer stroke than the 4.8L, but they had the same bore so they shared a common block. All of the ’99-’03 engines and most of the ’04s came with the 12551358 cast iron block, but there were a few that had an aluminum block.