You CAN increase the diam of aluminum/silicon piston by heating it to high temp. Then you have to drop it in very high heat state to cold water, prefarably salt water. It will "freeze" in expanded state. I give you here another hint on cold/hot massge: it is about tuned pipes or spun exhaust mufflers. If you have bad dent, don`t put any hard object in it to bend the dent out. Just fill with water almost full. close then the ends with paper towel plug and put in a freezer. When water becomes ice.it expands and the dent pops out, nicely and smoothly, without any bad marks. Then just polish it with aluminum polish and yo can sell the pipe as almost as new...