Pistons and Cylinders are pretty straight forward. After you take the head off try to move the piston, sideways, in the bore. There should be almost no movement. look at the cylinder. If there is an obvious ridge at the top of the cylinder and you can feel it with your finger, and there is piston movement sideways, you are going to have to bore it oversize. The factory oversizes are in quarter millimeter increments. Like .25,.50,.75,1.0 and the like. .25 mm is about .010". That's enough to clean up the bore, but it is not enough to increase the size of the engine by much. If you are unsure, measure the diameter of the bore and the diameter of the piston with inside and outside micrometers. Measure the piston about 1/2" up from the bottom of the piston skirt. Now measure the cylinder bore in several places at right angles to each other. Subtract piston diameter from the biggest bore diameter. This gives you the piston clearance. Piston clearance varies due to a number of factors.
- Diameter of the piston.
- Type of metal the piston is made of.
- Is it a cast or forged piston.
- Is the engine air or water cooled.
- Is the engine two or four stroke.
A small (50mm bore) cast piston, 4/stroke may have half a thou (.0005" or .0127mm ) piston clearance, while a big, forged piston, 2/stroke (73mm, 2.874") can need .003" (.075mm) or more. You will need a shop manual to find the clearance for your bike. Now this is the minimum. What is the maximum ? I once had the exhaust off of a 400cc 2/stroke single, and I was able to slip a .020" feeler gauge between the piston and cylinder. Why did I have the pipe off ? The guy wanted me to check the pipe out. I found several, large, rubber, air cleaner box mounts in the pipe ! The guy said he had never had the pipe off. The bike ran for several more months before the piston skirt broke. Bottom line, get a shop manual and get the right clearance. If it is an after market piston, use the piston manufacturers clearance recommendation. 
If your old piston is within spec, and you want to reuse it, you will need to measure the ring side gap. Side clearance (gap) is measured with a feeler gauge, with the ring in it's groove in the piston. Just try to fit the feeler gauge between the ring and the top or bottom of the groove. They can be .001" to .003"(.025mm to .0762mm, check the manual). If you are using a new piston and rings you do not have to check this. If the gap is too much, even with new rings, you will need to replace the piston. Everybody seems to think a new set of rings should cure everything. I wish it was true... but it's not ! I would say 95 % of the time, on a dirt bike, you will need to bore it oversize. Sometimes, on a big street bike, the pistons will be within spec, but on most of the dirt bikes you are going to have to bore it.
It is best to have the cylinder bored on a boring machine. I have honed them as big as .040" over and I held it to spec on taper and out of roundness, but it took forever. Now, if I go bigger then 1/4mm or .010" I like to bore them on a machine. If you do try to hone them be sure to use a good hone like a Sunnen or Ammco. You will have to measure the piston and then bore the cyl to fit. Do not bore the cyl unless you have the piston to measure. If you do, the clearance may be off.
Remove the circlip and push the wrist pin out with a screw driver. You may lightly tap it but if the pin is in too tight, you will need to pull it with a wristpin puller.
If the piston has seized and smeared aluminum on the cylinder you will need to remove it before you measure it. You can do this with Muriatic acid. Just take medicine dropper and put just a few drops on the aluminum that has been smeared in the cylinder. The acid will eat the aluminum but will not hurt the steel of the cylinder. DO NOT GET THE ACID ON YOU . If you do get it on yourself, water and baking soda will get it off. Also don't get it on any aluminum unless you want that aluminum eaten up ! You can get Muriatic Acid at any home improvement store. They sell it buy the gallon, which will do about a billion cylinders. I'm not sure what they use it for... swimming pools or something.
If you have a 2/stroke, be sure to look at the top of the piston. If it looks like a rat has been chewing on it watch out. It means that the bearings in the bottom end are starting to go. If you put in a new piston it will just be ruined as more bits of bearing shed by the lower crank and rod bearings. The cure ? Rebuild the bottom end. Be sure to check the cyl for any rust pits. If there are any you will need to bore the cyl to a bigger over size to get rid of them. If you are restoring a machine, you will find these rust pits are wherever the rings stopped in the cyl. A lot of water is created during combustion. Supposedly, about a gallon of water for every gallon of gas burned. Some of that water tends to collect around the rings.
When you get your cyl back always ask if they cleaned it. Most places do but, you never know. To check, take a clean rag, put some fresh oil on it and wipe the inside of the cyl with it. If it brings up any dirt, take your cyl and wash it in lots of hot soapy water. Immediately dry and oil the cyl so it will not rust.
