there are a few ways to do it, but the best way is to follow the instructions for that specific motor. it will come with it, but the Idea is to heat up the piston and sleeve so they form fit is you will. the main point is to get your motor to be around 220F for a while, then let it cool (make sure the piston is not at the top during the cooling part). there are a lot of topics on this, do a search and you will get a lot more out of it. this is a very important step as it is what makes or breaks your motor for the long haul. following the instructions is important to make sure you get a full warranty just in case. this is why a temp gun is a good thing to have. you will also see that some sue the "spit test" where you spit on the head and see how long it takes to boil and how long it lasts. while this will work, it is not a precise.