Most people don't have the proper tools to measure their piston to cylinder clearance here is a quick and dirty way using a feeler gauge, first a few things you might want to know.

Piston to cylinder clearance is the difference between the largest inside diameter of the cylinder and the largest outside diameter of the piston, the difference between these two is the piston to cylinder clearance.

A little about these 2 stroke pistons, the piston is tapered from top to bottom the piston is smaller at the top and bigger at the bottom, the piston is also NOT ROUND it is "cam ground" or machined as a oval, the piston is bigger on the thrust sides (exhaust and intake sides) than on the wrist pin hole side.

The cylinder bore should be round.

They machine these pistons smaller on the top because that is where most the heat is, the top of the piston will see 1200 to 1300 degrees F when it does the aluminum expands this is why you need the clearance so you have room for the film of oil that keeps your piston from sticking to the cylinder wall from friction.

Engineers say that once the Engine is up to full operating temperature and running under full load if you could freeze the size of the piston remove and measure it the piston would measure round, its all about gaps and allowances.

Back to measuring the clearances first clean the piston and cylinder, you wont need the rings on the piston so remove them, then turn the cylinder over you want the bottom of the cylinder facing up.

Next you want to slide the piston into the bottom of the cylinder, you want the top of the piston to slide into the cylinder just like your installing it in the Engine the top up, you want the exhaust side of the piston on the exhaust side of the cylinder.

As you slide the piston in the cylinder you also want to have a feeler guage between the piston skirt and cylinder, start with a thin feeler gauge like .002 thick, very little force will be needed their should only be a light resistance as you try to push the piston through the hole with the feeler gauge between the piston and the cylinder, if their is little to no resistance then try it with a .003 you keep trying thicker gauges until its a real snug fit, in this case if the .003 is too tight then try a .0025, note not all feeler gauge sets have the in between sizes.

Each make and model of Engine has a different piston to cylinder clearance each piston manufacture has its own clearances so if your using OEM then use the clearance in the service manual, if using the Wiseco pistons use what they tell you for clearance.

Note SOME people and tuning books will tell you modified engines require MORE clearance than what the factory specifies this might be true on some engines, I can tell you the Pilot factory clearance for the OEM piston is .003 and even on my full modified engines all I have ever used was .003 with no problems, I set all the Wisecos to .0045 as per instructed by a few Engine builders and a guy from Wiseco.


Hope this helps, any questions please ask.

I strongly suggest you read the service manual it clearly shows you how to inspect and measure your piston to cylinder clearance this is just a short cut for the average guy that does not have all the fancy tools, the original idea of this post was so EVERYBODY can take a quick check on the clearance before assembling, I have ran into MANY engines over the years that had .008 to as much as .014 piston to cylinder clearance hopefully this will help prevent a Engine with way too much clearance from ever being assembled and ran.

UPDATE After some questioning by Pro-Pilot-Popper this was rewritten so thanks goes to Pro-Pilot-Popper for making me realize I had left out a bunch of info I should have included the first time it was written.

That is a quick and "dirty" method but it should also be taken at TDC (Top Dead Center) as this is where most cylinder wear can be found. Compare the measurements from TDC (Top Dead Center) to BDC subtract, and the difference will be your taper of your cylinder. Just taking a measurment at BDC will give you a false measurment of piston to wall clearance and IF it is at the outermost end of the spec at BDC then you may have a piston slap issue in which a piston skirt will break/crack.

All my years of checking cylinders this way have shown me when the bottom of the cylinder as I have checked in the pics is good so is the top of the cylinder, when its bad so is the top, I always measure this way as a preliminary measurement then measure using a dial bore gauge or with a snap gauge and calipers.

Try it, find a cylinder that was taken off a running Engine and didnt suffer a broken piston or seizure that is worn out at the top of the cylinder but still measures good at the bottom as I am showing in the pics, I have never seen one.

don't forget that a cylinder with any wear at all their is always a area that the piston or rings never touch at the very top of the cylinder, if the bore is not warpped you can actually measure and find the original bore size in this area, if I do my quick and dirty test at the top of the cylinder that has a area not worn I get a false reading, follow me? On a 4 stroke you use a ridge reamer to remove this area.

You can see the area I am talking about on the bling machine cylinder the blue arrow

You wouldn't take a measurment at the ridge, as you said there is no wear at that point. Take your reading an inch or so below the ridge mark and then another at 90 degrees from that measurement. This perpendicular measurment is also very important in determining if you have an egg shaped bore. This becomes a common wear pattern on stroked engines where the rod angle is enough to "push" the piston to one side of the cylinder. Make this measurment, both at the top and bottom of the cylinder. This is why you can not use dial calipers to get an accurate bore reading, they don't go down past the ridge. If you are removing metal when using the ridge reamer you are not using it properly. It should just lightly scrape the carbon off, then use a scotch brite pad and solvent to remove the lighter stuff under the crust. Tapered cylinders are very very common higher mileage engines. Often the cylinder is shiny and glass like in appearence. Remove the piston and look at were the piston is at BDC, you will most likely see that you still have a hone pattern in this area. I'll see if I can remember to bring the camera to work and snap some pics for you.

My ridge reamer comment was to reference the area of the bore I was talking about I was not implying you use it to remove metal, if you needed to remove metal the piston would have never fit when new heh.




I think we are getting off topic here my original post is to aid a guy that has a CYLINDER a PISTON and a FEELER GAUGE SET for measuring tools, the idea is to take a QUICK and DIRTY measurement of his cylinder.

The instructions are for a guy that got his cylinder back form being bored and wants to see if the guy that did the machine work gave him about .0031 clearance or .014 clearance.

I have found this method accurate to with in .0005 its also a quick and dirty method to see how much your clearance is on a used piston and bore.

Your performing this mostly by 'feel' if the very top of the bore is not worn it will give a false feeling when you push the piston down into the bore and the piston skirt gets bigger, follow me?

"H" thanks for the tip. I'll give it a shot next time just out of curiousity. Whe using the tip would fliping the piston over when measuring the top of the bore (always measuring from the skirt) be more consistant using the skirt as a constant. Some times the skirt as a larger dia the the crown. This would be a good field use tip. Rering or not in the field for those who have a c cylinder and and a,b,c pistons,rings and hone in there tool box.

I am not sure I follow you.

If you would be replacing parts then you need to measure with more accurate tools remember this is a quick and dirty method.


All 2 cycle pistons should be tapered, smaller at the top than the bottom, they also should be oval not round, so the skirt or bottom of the piston should always be larger than the top.

hers a hypo. let say I have an "C" cylinder away from home and spare parts in the box. get up for the morning ride and do comp check and notice its down 20 to 30 lb I put in a cap full of two cycle oil and it comes back up. Yea bad rings, stuck rings, colapsed ring, ect. Pull jug leaving head gasket on. No no stuck rings on piston and no burned up piston. pull rings stick in bore and are out of spec. pull piston and head gasket off. use dirty method to check cylinder wear( remember I left my goo set of tools at home) yea it check with limits. Get out hone, champfer and re ring and run for the duration of trip, Or not in spec ( piston to cylinder clearence) and not out of round hone for next size piston"B" and and check ring cap if ok install for duration of trip. I always use ring method but was willing to try dirty on next tare down to see what I come up with. I also have spare cylinder ready to go for long trips. I collect old but good A,B, and C pistons for spares and prep work and sets of new rings and wristpins ect. for emergency purposes. If all that don't work break out other bike. If I'am off tract please set me up right. Gauges are the way to go if not have a friend that does. Sorry for the babble.

O yea the turn it over meant, use the piston skirt turned into cylinder lined up correctly up the as a consent to measeure top mid and bottom of cylinder for dirty method clearence check piston to bore before making decision on what to do. Rering or new piston up one size C to B ,B to A with new rings ect. In field means long trip away from home and no shop close for bore.

I never use the quick and dirty method when replacing parts I always measure with precision tools.

Would you mind posting each of the proper tools.

I have feeler gauges etc. But would like to step it up to the next level.

What should I look for and whose should I stay away from.

thanks

I use mostly my dads tools he as a tool and die maker.

Basic machinist measuring tools, if you page through your service manual they pretty much show you how to measure.

Dial Calipers



Telescoping Gages





Dial bore gauge



Micrometer Set




This place is close to me, they stock name brand $$$ stuff and cheap Asian knock off china crap http://www.lostcreekmachine.com I never dealt with them over the web or phone always face to face they seem honest and hard working, they are farmers also, the are a family own operation.

perfect thanks.

My dad was a Mechanical Engineer and my bro has all these tools of his since he is a ME as well. Management now.

MasOdy was asking about Engine measurment in the chat here is some info and what I do.

Below are the tools I use, left is the mic top is dial bore gauge, below that digital calipers bottom veneer calipers, little square thing with the hole in it is a standard, its exactly .100 thick, it came from a certified set.

First I take the piston and a mic and find the biggest part of the pistons outside diameter, this is usually found at the bottom of the piston skirt on the thrust sides.

My mic holder is nothing fancy I just use my bench vice with soft jaws (old towel) to hold the mic whilst I put the dial bore gauge in the caliper .

Next I zero the indicator on the dial bore gauge.

Put the bore gauge into the cylinder at the top on the thrust sides and move it up and down take readings, your looking for the largest OD of the cylinder this is usually found above the exhaust where the piston rocks as the thrust loads change in the cylinder bore.

The largest I found was slightly under .010 that means the piston to cylinder clearance is .010 of you wanted a closer to exact number I would say its .0098 on this indicator each line is .0005

You can also use a telescope gauge to measure the ID of the cylinder this takes longer than using the dial bore gauge, I use both to double check my work, I do this because I don't use these measuring tools daily so like to double check myself as much as possible.

You put the telescope gauge in the cylinder and tilt it slightly so its length is longer than the bore is wide, you snug the locking device on the end, you gently and slowly rock the telescope gauge so it gets tighter in the bore, this movement will shorten the telescope tool to match the bore ID, you slowly and carefully tilt it so it can be removed from the bore.

You then carefully check its length using a mic

I double check using a calipers too

I check my standard before and after to ensure my measuring tools have not changed, they never do its just good work practices my dad taught me when I was a kid.

It only takes a few seconds to perform the test so why not

Really stupid question here Hoser, what if the clearnace is like you said .010 at the exhaust, but father up the cylinder it is at .003, you gonna have to hone that cylinder out anyway due to the .010 correct so it wouldn't matter if the top of the jugs clearance is fine correct?