A very delicate and important aspect of engine tuning is setting the piston-cylinder tolerance. This determines variations both in performance and in reliability of the engine.
How tolerance acts
The extremely wide range of functioning of two-stroke competition engines, from 7000 to 20000 revs per minute, determine the necessity to work on extremely precise tolerances between all engine components, in particular between cylinder and piston.
Such tolerance represents the space present between the internal side wall of the cylinder and the external lateral surface of the piston. Such space is extremely small, more or less around some 10 hundredth of a millimeter, and variations of a hundredth of a millimeter make some difference in engine performance and reliability.
The function of the piston, as we know, is to transform the force given by the high pressure of the burned gases obtained by combustion into movement. The piston does this with good efficiency if gases are not allowed from the combustion chamber through the space between piston and cylinder into the crank case, before the exhaust port opens.
At low revs the piston of course moves at a lower speed up and down inside the cylinder. Slow speed gives more time to the burned gases to make their way in such passage. This means that a greater value of the tolerance between piston and cylinder determines more pressure loss, and lower torque value. So a lower value of tolerance is needed, since low revs also determine a less critic movement of the piston inside the cylinder.
At high revs the piston has a higher speed and burned gases really have not much time to pass through the space between piston and cylinder. Tolerance can be greater and such greater value helps the movement of the piston inside the cylinder since friction is reduced with greater tolerance values.
Friction also determines the production of heat together with the erasure of the walls both of the cylinder and piston. Friction produces heat and a force acting on the piston opposite to the piston movement. This means that a low tolerance generates friction reducing maximum revs and increasing engine temperatures, with great danger for what regards engine reliability. If, in fact, engine temperature increases over a certain limit detonation (small explosions inside the combustion chamber) can occur and this means also probable engine seizure.
Of course when the engine heats up also the cylinder and piston heat up. This means that both components will expand and diameters of will increase. Generally though it is the cylinder that will expand more so tolerances will increase. On the other hand both components will expand in a not uniform way. For example both components will heat up more in the upper area since it is the area where combustion occurs. This is why both cylinder and piston are slightly conical. Also the cylinder with its various ports will deform itself and the small areas of cylinder between two ports, like the areas between the exhaust port and the boost ports, will somehow expand towards the internal part of the cylinder reducing the diameter in that particular area. This will generate a strong reduction in tolerance between piston and cylinder and a risk of engine damage and seizure. In particular you will notice that there will be extremely polished areas on piston and cylinder surface in the zones where the exhaust port and the boosts are close to each other.
Temperature also means that with different weather tolerance could need to be adapted. What happens usually is that high temperatures tend to increase the risk of engine seizure and low tolerances tend to increase heat production by the engine. This means that with hot weather tolerances should be greater. On the other hand hot temperature means more expansion of the cylinder and increase in tolerance, which leads to loss of torque at low revs. To this we must add that hot air is less dense, which means less mixture enters the engine and power output is reduced. So hot conditions are quite a complicated situation.
First of all if you need to tune your engine you should know what the basic tolerance between piston and cylinder should be. Your engine builder should give you a good indication. Of course such value will generally be higher then the tolerance “of excellence”, because more tolerance means better reliability of the engine. Anyway start from the value given by the constructor and then try to reduce such value of around 0,01 millimeters. Generally a medium value to start from is 0,10 millimeters.
Consider, as seen in the previous paragraph, that if you reduce tolerance you will obtain better torque at low revs, but also a reduction of maximum revs, which means lower speed along fast sections of the track. Consider what kind of track you will be running the engine in, then make your choice. Be careful in reducing too much tolerance because you will risk engine seizure, and with low tolerance always consider a longer running in of the engine, since the piston will need more time to adapt to the cylinder internal wall.
Looking at a new piston you will see that a layer of graphite (grey) will be present on the piston lateral surface. This graphite will help running in by adapting to the cylinder shape and wearing out where tolerance is slightly less compared to other areas.
Practical measurement of tolerance
We will need all of next issue’s column on engine tuning to go deep into practically regulating tolerance between piston and cylinder. Precise instruments such as Palmer caliper and centesimal comparator will be needed. But also important hints on temperatures of piston and cylinder when measured will have to be considered.