Phase One The first thing you need to do is all the obvious stuff:
1) Remove all the plastic in the way. In some cases you may need to remove the gas tank also.
2) Remove the exhaust pipe.
3) Remove the carb and reed cage.
4) Drain the radiator and remove the hoses. PLEASE remember it's not just water - it's antifreeze. Dispose of it properly.
5) Remove the spark plug and any brackets connecting the head and or cylinder to the frame. Now that all the prep work is done, let's jump right into our rebuild. PLEASE remember that once you open the motor up, it is very important that NO dirt should be allowed to get into the motor. 1) Remove the head from the cylinder by first removing the nuts then lifting the head up off the cylinder. First, when removing the head nuts, it should be made clear that the untorquing should be done gradually and in a pattern, so as not to risk warping the head. The same goes for torquing things back together; it should be done evenly and in the pattern recommended by the manufacturer. Also, never use a screwdriver or other device to pry off the head or cylinder. You can use a rubber mallet to free up a stubborn cylinder.
2) Remove the cylinder from the motor by removing the nuts, (and yes, you should gradually untorque and retorque the nuts using the pattern recommended by the manufacturer) then lifting the cylinder up off the studs. PLEASE note that on some motors there may be other procedures that may need to be preformed before lifting the cylinder off. Example: on the Suzuki 250 & 500s have a cover that must be removed to expose a link that is connected to an exhaust valve. One end of the link MUST be disconnected.
3) Once the cylinder has been removed, EVERYTHING (reed cage, Suzuki exhaust valve, old gaskets) MUST be removed from the cylinder.
4) Remove the old piston by removing one of the clips holding in the wrist pin, then pull the pin out. Be careful not to drop the washers or bearings into the motor as you pull the wrist pin out of the piston. One way to guarantee nothing gets in the bottom end is to stick a clean shop rag over the crank to keep junk out

Phase Two Once the motor has been disassembled, and only after the motor has been disassembled, you can now determine what size the new piston will be. To determine the size of the new piston you must measure the size of the cylinder bore. You do this with a cylinder bore gauge. If you do not have the proper measuring instrument, you can take the cylinder to a bike shop or a engine rebuild shop to have it measured. Once the size has been determined, you buy a piston and set of rings the next size up. If you clean off the top of the piston and there are no numbers on the top, it may be a stock size piston. If it has numbers on it, those numbers can tell you what size the piston is. If it has a number .25mm on it, it means it's a "10 over" piston. If it has a number .50mm on, it means it's a "20 over" piston,and so on and so on. The reason for buying a piston the next size up is because you need to have the cylinder bored to match the new piston and rings in order to have good compression in the motor. Once you have the new parts (piston, rings, wrist pin, wrist pin bearing, gaskets), you should then take the cylinder, rings and piston to an engine rebuilder to have the cylinder bored to match the new piston and rings. NOTE: Please be sure the cylinder is completely disassembled. Example: In the case of a Suzuki LT250 or LT500 the exhaust valve MUST be removed. If any of the grit from the boring and honing process gets into the valve or bearings, it will be damaged. You can take the cylinder to a bike shop to have the work done, but I find that the local automotive engine rebuilder in my neighborhood will do it cheaper and faster. Once the cylinder is bored to match the new piston you will need to make sure the cylinder is spotless before you can reinstall it. Rinse it with warm soapy water, then with clear water. Dry the cylinder then spray it with a very light coat of WD40.

Phase Three You are now ready for reassembly. It is simply the reverse order of dissasembly, with just a few things to be aware of. Always read the instructions that come in the box of your new piston. There may be special instructions that pertain specifically to your motor that may be very important. 1) Install the piston rings on the piston. The instructions in the box will explain exactly how they fit.
2) Attach the piston to the connecting rod.
a) Slide the bearing into the connecting rod.
b) Place the bearing spacers at each end of the bearing.
c) Slip the piston over the bearing.
d) Then slide the wrist pin through the piston and bearing assembly.
e) Install the wrist pin retainer clips.
It should be noted that while the piston will fit on either way, you need to make sure that it is installed per the instructions that came in the box. That will usually be with the mark (or arrow) on the top of the piston pointed toward the exhaust port. If the piston is installed improperly, damage may occur.
3) Coat the piston with a light coat of 2 stroke oil.
4) Install base gasket. Make sure ALL of the old gasket was removed from the mating surfaces or you WILL have an air leak.
5) It is now time to slip the cylinder over the piston. You must make sure that the rings are in their correct position and that they are not catching on any thing as you slide the cylinder over the piston. If the cylinder does not go on smoothly, check to make sure the rings have not moved. DO NOT force the cylinder on. Damage will result, at your expense.
6) Torque down the cylinder as per the manufacturer's specifications.
7) Place new head gasket into position.
8) Place head into position and torque to manufactures specifications.
9) At this point you are ready to reassemble all of the misc. things you had to remove. Such as.....
a) Reed cage and carb
b) Exhaust pipe
c) Radiator hoses (don't forget to refill the radiator with the proper mix)
d) Any linkage that had to be removed, including any engine braces.
e) Gas tank and plastic
f) Install a new plug
10) Double check everything !!! Now that you're ready to fire this Bad Boy up, remember, break it in easy, so here's a Break in page to make sure this new motor will last. At idle, a cold 2-stroke doesn't get enough top end lubrication because the cold fuel/air mix condenses in the bottom end and doesn't make it to the piston/cylinder walls. The motor should be GENTLY revved up and down to make sure the cylinder gets lubed properly. Do this and the motor will last you a long time. Special thanks to Jason Witzeling and Colin Bylund for their input. Thanks Guys!!