there are two reasons to use aluminum over brass, one is weight! the second reason is that aluminum sleeves are a radically different alloy than the piston, the pistons contain 20-30 percent silicon....and no they don't expand at the same rate as the sleeve, they actually expand very little, where the sleeve expands more rapidly...aluminum sleeves seldom bellmouth like brass ones...hence the sleeve shrinking dies are sold for brass sleeves..Dub Jett addresses this on his companies website, and comments that often a new piston is all that is needed on engines after long use with aluminum sleeved engines..aluminum sleeves are easier to machine as well as transfer heat faster than brass sleeves do.