Yes, the shims are used to adjust lifter preload. Some say they really aren't a good idea because they effectively increase the diameter of the rocker shaft, which makes them too snug for the pedestals in the head and causes them to crack. I've never actually seen one crack, so I'm not sure if it's true, but it sounds logical. Adjustable rockers are better, but expensive. If you want to use the stock rockers, the correct way to adjust preload is to get a custom set of pushrods made. That's what I did with mine.