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 .0010". 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.
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 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 where ever 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.

The piston's rings must have a small gap between the ends when they are installed in the cyl. This end gap is given in the shop manual and can vary quite a lot. Like .006" to .020"(.15mm to .08mm). End gap is measured by putting the ring to be measured in the cyl and pushing it up with the piston to within 1/2" of the top of the cyl. By pushing it with the piston, the ring is squared up in the cyl bore. Now you can easily measure the end gap. If the gap is too small, file a bit on the end of the ring (Carefully !). Do only a little at a time, until you are within spec. If it is too big... well... ah... tough !

Side clearance 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). If you are using a new piston and rings you do not have to check this.

When I install a new ring use I like to use a ring expander if I can. If you don't have one you can do it with your fingers, just be careful not to spread the ring open too far or bend it sideways too much. Rings are very hard and brittle and break easily. Always install rings with the markings on the rings up. A word here on rings. There are many different styles and types of rings. Dykes, Keystone, Standard, you name it. You must replace any ring with the same type and size. Some people think that the spring pressure exerted by the ring on the cyl walls is what seals in the combustion pressure. That is not true. Think about it. Combustion produces several thousands of pounds of pressure. You can compress the ring with one hand, right ? It is that combustion pressure itself that channels down between the piston and ring, pushing the ring against the cyl wall. So don't put a .5 over ring in a .25 over cyl and think you are helping, your not. In fact, that ring will not contact the cyl wall right. So you could be losing compression. Another thing, after you install all the rings, with the markings up, stagger the ring end gaps about 1/3 or 1/2 apart so the gaps are not in line with each other. If you have a 3 piece oil ring stagger them this way. On four strokes, you can have a one piece oil rig or a three piece oil ring. the three piece ones seem to work better but are harder to get in the cylinder.

Before you install the piston, oil the piston and cyl up real good. Move the rings around in their grooves and make sure there is plenty of oil on all the parts. Now you are ready to install the piston. Pistons do have a front and a back. If there is a mark or arrow on the piston it always goes to the front or exhaust side. On 4/strokes there may just be an "IN" in one of the valve pockets. This goes to the intake side. (Duh !) On most cylinders I like to use a Ring Compressor. There are lots of different types of ring compressors. You can even use a regular hose clamp. The piston and rings should go on smoothly with just a little tap with a piece of wood. They say you should be able to push the piston and rings on with your thumbs... but I never can. Bad thumbs I guess. So I use a bit of wood or a lead hammer very gently to tap it in. Do not force it. If it just will not go, reposition the ring compressor. With 3 piece oil rings you just about have to use a ring compressor. If the cylinder is a big port 2/stroke you may have to reach in through the intake port, with a long thin screw driver, and push the ring so it does not get caught in the port. Also remember, on a 2/stroke, to position the ring ends to the peg in each ring groove.

I like to put the piston with rings and one circlip installed, in from the top of the cylinder and push it down till the wrist pin hole is uncovered. Then I slide the cyl down to the rod (and bearing on a 2/stroke) and then push in the wrist pin. Install the circlip and push the cyl the rest of the way down. Don't forget the base gasket. Before I install the circlip I like to put a rag in any hole the circlip could go down if it gets away from me during instillation. I always use new wrist pins, wristpin bearings, and circlips, although I can live with reusing a circlip. Replace with new, everything else.

Two, three and four cyl engines usually have the crank pins staggered so you can put the pistons in one or two at a time... makes for lots of fun. Go slow and they will go in !

Piston oversizes are usually marked on the piston. Trouble is, sometimes the markings are printed on with ink and not stamped in or they are worn off. I always measure the old piston so I can order the right oversize.

Something to remember, if you are working on an old bike and are having trouble finding a piston. As long as the wrist pin diameter is the same and you can bore the cyl to fit it. Almost any piston will work. You might have to cut the piston skirt down or drill big ports in it. You might have to grind the head to fit the combustion chamber or grind inside the skirt to make it the right weight, but with some imagination you can make it fit and it will work.

Wow, I guess there is a bit more to pistons then I first thought !