Hi All, I often see posts about having to do top end overhauls on the forum and I thought it might be useful to give some information on this. Most of this is based on my own experience and I will provide links for other sources. Sorry, but I have zero flexibility when it comes to the tolerances and methods that I discuss here. This is controversial and what can I say? Maybe take it all with a grain of salt. I have thought about this for a long time and I figure that it is best to help a few people out and just say sorry to the others. No offence is intended. Personally, this area of engine rebuilding takes me an awful lot of time, so I will try to cover just a few of the important points that relate to fitting new rings. When it comes to a smooth and clean running powerful engine there is not much that is more important than a good piston to cylinder seal. I firmly believe that few people understand how important this is to engine performance. I will make reference to this point later. This seal can be long lasting and reliable but it is a very fine line between good and terrible. To create this seal the rings need to be bedded into a finely honed cylinder bore. I never imagined that I could find a pic of something as bad as this. This is not a finely honed cylinder bore. This engine is ruined even before it is assembled. It will never run well and by the time it does run in, it will be worn out. If you walk into an engine reconditioning shop and you see something like this then do a u-turn and walk away. This is more like it. This has the makings of a good reliable engine, but I still prefer a finer cross hatch than this, and I am trying to find a suitable example. I know I saw a good one from the new Norton factory, but I can't find it now. And of course technology marches forward at what seems like an ever increasing rate. This is laser honing from http://www.gehring.de/en/technology/laser-honing/. You can see that after the etching process it is finished with a superfine hone. So let's begin with the piston and bore. For the sake of this article I will use the information from a Kawasaki ZXR250 workshop manual. Note that the manual does not give the maximum piston-skirt to cylinder bore clearance, or wear limit. Using the maximum limit on both items would yield 0.29mm. Given that this is a very large clearance, I would strongly recommend that you strictly follow the wear limits for the piston and bore separately. To even begin to work with this level of precision requires accurate instruments. The bore inside diameter is specified down to microns, or thousandths of a millimetre. Those values are so fine that they will be affected by ambient temperature. Fortunately though, we are only dealing with the fitting of new rings so we can get away with some relatively inexpensive equipment. Or, as very wisely suggested by a forum member here, make use of your local machine shop. They can measure your parts for you very easily and for a nominal fee. Cleaning: It is assumed that you have disassembled the engine as necessary. The first step is cleaning. There are many ways to do this but you must be very careful with the pistons. The ring lands, or grooves, are very precise and must remain parallel and within tolerances. Physically scraping carbon deposits out of them should be avoided for that reason. I would suggest ultrasonic cleaning or soaking in a solution that is specifically designed to dissolve carbon deposits without damaging aluminium. Removing the piston pins should be done with the factory tool or similar (not hammered out). Once they are clean the process of measuring can begin. I suggest that after the horrible cleaning work is finished, that you stop work and make yourself a nice, comfortable, well lit and clean area to work in. Make a sign “No Filth Permitted Past this Point”. Piston Diameter: The point at which the piston skirt diameter is measured is described in the service manual, in this case 5 mm from the base (always perpendicular to the piston pin), and of course this should be done with a micrometer. This is because pistons are tapered from their smallest diameter, at the top, down to their base. Cylinder Bores: Again the positions at which the bores should be measured are described in the manual for your model. In the case of the ZXR250, they are, 10mm and 60mm from the top edge in both fore-aft and left-right directions for a total of four measurements. Unfortunately the workshop manual does not specify a value for maximum taper nor ovality. Well, not one that I can find at least. However, the Honda workshop manual for the CBR250 gives a limit of 0.05mm and this seems like a reasonable value. Since we are only hoping to replace the piston rings these measurements can be performed with a telescoping gauge and a micrometer, not a digital vernier. Even the best vernier calipers, (and that includes digital ones) are only accurate to 0.02mm, which is unacceptable. Metric micrometers are dead easy to use and are not expensive. Taper: This is the difference in value between the two points, the first 10mm from the top edge of the cylinder and the second 60mm, in the same axis. As mentioned I suggest 0.05mm as a maximum. Since there are four measurement points you are making two comparisons. Ovality: This is the difference in value between the two axis. One parallel to the piston pin and the other perpendicular. As before there are two point of comparison, the first 10mm from the edge and the second 60mm. Again I suggest 0.05mm as a maximum. Clearance: Now for each bore subtract the piston diameter from the largest diameter that you have for that cylinder in the perpendicular to piston pin axis. This is your piston-cylinder clearance. See the 1st table above for the details. Small End: Again I cannot find the section in the Kawasaki manual for these measurements. More work to be done. Cylinder Deglazing: Now we are entering the twilight zone. I am taking about deglazing a cast iron cylinder sleeve here not a Nickasil coated aluminum one. Please refer to this document http://www.brushresearch.com/pdf/NOP.pdf I first read this in 1975 (yes, that is right, no internet) and have used Flex-Hones for deglazing cylinders since. There is also other technical documentation from the same source http://www.brushresearch.com/literature.php?type=2 Use a 320 grit Flex-Hone to deglaze the cylinders (This is quite a light grit, more on that later). I attach to a drill and I have done so many of these that I just know what speed, and the rate of up and down motion to use. I run slower than is shown in the videos. Do a few strokes to get your cross hatch angle right first. You will get the hang of it quickly as it is not difficult. Follow the instructions to the letter and watch the videos. I like to use 10 weight oil, ATF is fine. After honing clean, clean and clean again. Edit: Murdo and Risky prompted me about this important point, as advised in the instructions, you need to test that the bores are totally clean from residual grit. I use paper towels and Murdo and Risky have suggested their techniques, all good. Brush Research, Sunnen and every other credible source that I have seen, advise to clean with hot soapy water, and they know more than you or I. Piston Rings: Piston rings are made to suit the corresponding size pistons. One does not try to stuff oversized rings into a standard sized bore, so if you have a standard set of pistons you will be buying a new set of standard rings. Step one is to measure the thickness of the new rings that you have bought to make sure that they are not duds. Gapping the Rings: You need to fit the rings into the bore and use the piston to gently push the ring squarely down into the bore. Find the point at which the ring gap is the smallest and this is where you will continue to check the gap. Remove the ring and adjust the gap by filing the ends. Take the smallest amount off then check again. Since you have an old bore, you will probably not have to increase the ring gap as it will already be larger than the minimum value. Ring Groove Clearance: Next fit the rings to your perfectly clean pistons and use a feeler gauges to measure the ring/groove clearance. It must be the same all the way around the groove. To quote the workshop manual “The rings should fit perfectly parallel to groove surfaces. If not, the piston must be replaced”. Assembly: Follow the workshop manual. Get a friend to help you fit the cylinder block over the pistons. You do not have enough hands. Be patient and methodical. It is going to take a lot of time. You will not be able to “throw it together”. If you need a tool then stop work and buy it or make it. Running In: Here is the nasty part. Sorry, you are not going to run in your new rings. You are going to give your bike a good run. Here are some references. I do not necessarily agree with everything they say and there is a bit of variation in views but I am sure you will get the gist.. http://www.mobiloil.com/USA-English/MotorOil/Synthetics/Synthetic_Oils_FAQs.aspx MYTH: You should break in your engine with conventional oil, then switch to a synthetic like Mobil 1™ oil. http://www.cbr250.net/forum/cbr250-performance/612-engine-break-myth-pros-respond.html http://www.motorcycleextremist.com/Motorcycle-Engine-Break-in-the-Right-Way!.html http://www.mototuneusa.com/break_in_secrets.htm You have a very small window of opportunity to make that 'seal' that is so precious to engine performance. The one that I mentioned at the start of this article. Just how fine can that seal be? Well get your head around this. The factory tolerance for the piston/cylinder tolerance of a freshly bored and honed Kawasaki ZXR250 is . . . wait for it . . 10 Microns. That is one hundredth of a millimetre. For any Americans that might be reading, that is 0.00039 inches. That is the maximum variation allowed. You can get a cylinder bored and honed to that degree of accuracy at other places besides the Kawasaki factory but it is not easy. Now here is what I suggest. There are a couple of variables. The first is heat and the second is how badly worn was your cylinder? If it is a very hot day and your bore clearance is still quite tight then I would wait until it is cooler or night. We want to get the engine up to operating temp and use about 60% of rpm to do that. Do NOT ride in traffic with stoplights etc. Once the engine is up to full temp we want to do a few full runs up to redline in say 3rd and 4th gear. The engine is not working too hard but you are using the full rpm range. Remember you have a worn engine and you need these rings to bed into your fine cross hatch quickly (remember it was 400 grit). It is never going to happen if you trundle around. I would want to let the bike cool completely down and repeat this cycle about 4 times. So over a period of days would be good. That is it job done. Big Jets: Now that your bike has compression again and a good seal you will need to get rid of those “big jets”. This is my pet hate; some of you may have noticed already. Lots of owners riding around with big jets. Their bikes run like rubbish and all that sooty carbon is like pouring sand into your top end. Then they post pics of their black spark plugs on web forums to show other owners who also have sooty plugs and they all assure each other that this is good. Run the smallest jets that yield maximum power, not the biggest ones that you can stuff in there and stop posting pics of your sooty spark plugs on the internet please. There, I said it, and I feel better now cheers Blair More Information: Here is a great article on some developments in this field. http://www.enginebuildermag.com/2002/11/the-smooth-science-of-cylinder-honing/ Interesting to hear that what Brush Research have been saying for decades is now accepted. This article is talking about new cylinders not old worn ones. The case exists that if the cylinder is not too badly worn that starting with a coarser grit, say 240 and finishing with a 400 might be better than my suggestion of just using a 320. That could very well be the case, and I would not hesitate to do that if I thought that it was the best choice for my engine. But when it comes to recommending a procedure for the average owner who just wants to fit new rings, I prefer to stick to what I know has worked. Also here is CP Pistons guide, again this is for a freshly bored cylinder. https://www.cp-carrillo.com/files/1-Landing Pages/Pistons/faq/cp_instructionsheet_v4262012.pdf Piston Ring Tribology: http://www.vtt.fi/inf/pdf/tiedotteet/2002/T2178.pdf Assorted articles on honing: http://www.docs-engine.com/pdf/1/honing-cylinders.html The Smooth Science of Cylinder Honing - Engine Builder Magazine http://www.enginebuildermag.com/2002/11/the-smooth-science-of-cylinder-honing/ Break-in your piston rings: https://www.enginebuildermag.com/2018/05/how-to-break-in-your-piston-rings-the-right-way/ And for technical junkies: https://www.highpowermedia.com/RET-Monitor/tag/pistons-rings .