Question: do i have a 4.8 or 5.3?
Answer: If you see a flat-top piston, you've either got yourself a garden-variety 4.8L or a higher-output, Gen IV 5.3L. Time to check the crank. If you see casting number 12553482, you've got a 4.8L. The 5.3L should have the same crank casting number as other 3.62-stroke LS engines—12552216.