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Tolerances between piston and cylinder are of great importance for both the performance and reliability of an engine. Two-stroke race kart engines with high performance and high compression ratios of up to 16:1 require a good match between the two elements. Friction What is friction exactly? What happens at a microscopic level between the two surfaces of the piston and its cylinder? If we consider two surfaces sliding one on the other we know there is always friction. The two surfaces, even though they may be extremely well polished, are always microscopically rough. If we enlarge the image of the internal surface of a cylinder we can see how its profile is uneven and looks like a chain of mountains, with peaks and valleys. The same is true of the piston that is ‘lined’ horizontally so its surface is also visibly rough. The two parts in contact in a reciprocating movement rub the peaks of both surfaces together. These peaks will hit each other and often break because of the collisions.
Such contacts are known as friction and this phenomenon creates a force that opposes movement, meaning the reciprocating movement of the piston within the cylinder is slowed by friction. Also, breaking the microscopic peaks transforms the movement energy into heat energy and that is why when two bodies come in contact and rub together they heat up. Running in The breaking off of small particles from the peaks of the two surfaces is what we call wear. The two surfaces will become increasingly smooth and similar in shape to one another. This is extremely important when running in an engine with a new piston or a freshly honed cylinder. The two surfaces will be similar to a point but with careful initial running in, the two parts will deform to exactly the same shape for a perfect match. If the running in is not carried out correctly, for example too quickly or with too cold an engine, deformation will be incorrect and wear of the two parts will occur unevenly so the final match will not be as precise.
Tolerance Why must tolerances be so precise and why is it so important? Always consider that the cylinder and piston deform under the stresses from the heating and cooling of the engine. Deformation of the two elements, with different shapes and made of different materials, will be different. So we cannot only consider the tolerance between the two bodies when we measure them singularly but also when they are working, which means when the engine is running. A small tolerance of 0.10mm can decrease to nothing and can lead to too much rubbing together of the two contact surfaces. Excessive wear and heat production can quickly lead to engine seizure. Moreover, the small particles that come off the two surfaces, especially during running in (and this is why it must be done progressively and slowly), position themselves between the two elements reducing even more the tolerances. This, of course, can also lead to engine seizure. Lubrication To reduce the effects of friction, such as wear of piston and cylinder surfaces and excessive heat production, lubricants are used. Lubricants position their particles between two surfaces, reducing the impact of the microscopic peaks. In fact the aim of a lubricant is to produce a film preventing contact between the two metallic parts.
Heat from combustion and the extremely high revs of 2-stroke competition engines make such a task extremely complicated and that is why fully synthetic oils or, even better, castor oils are used. Castor in a lubricating oil produces a very tough film that covers all the metallic elements of the engine that will not come off. The weakness of castor oil is that when burnt it produces carbon deposits on parts such as pistons and piston rings which can become blocked. Lubricant adheres best to the two surfaces of the piston and cylinder if their surfaces are not completely smooth. That is why as mentioned before piston surfaces are ‘lined’ horizontally. Similarly, the cylinder’s internal surface will always have diagonal honing lines that will house the lubricant. Tolerances for reciprocating elements All elements that are part of the engine and have reciprocal movement must work at certain tolerances as we saw last month. Information about friction, running in, lubrication and tolerances is valid for all the other matching of elements in the engine such as conrod big end and crankshaft axle, conrod small end and piston pin, the rotating elements of the bearings and their cages and the bearing guide itself, the piston ring and the cylinder as well as the piston and cylinder.