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We can analyze a big difference in tuning possibilities after having studied in the last two issues the diverse solutions used for the surface of the cylinder: cast iron or chrome bore. With cast iron it is possible to machine the cylinder. This is done when the cylinder liner surface has to be renewed, after a seizure and also when the surface is worn down, by honing the cylinder. But the possibility to machine the liner surface, since it has no chrome cover, also gives the opportunity to work on the ports and ducts of the cylinder to improve gas and mixture flow. Let us see the route that the mixture follows from carburetor to combustion chamber. Once the mixture has entered the crank-case through the carburetor it is then pushed through the transfer ducts (generally three or five) into the combustion chamber, the volume closed between the piston crown, the liner and the cylinder head.
Then compression phase starts with the piston squeezing the mixture by moving upward along the cylinder and reducing the volume in the combustion chamber when all ports are closed. The spark plug lights up the mixture that burns quickly increasing the pressure in the combustion chamber by about ten times. Once the piston moves downwards, thanks to such pressure increase, and opens the exhaust ports the burned gases exit the cylinder through the exhaust duct. So when tuning the engine to optimize gas flow in the engine we must follow the flow from the crank case to the exhaust duct. When, in fact, the mixture moves from the crankcase through the transfer ducts the walls of the ducts must be possibly converging and without any sharp edge to avoid any turbulence which reduces gas flow in the ducts.
Since when the engine is mounted the single parts such as cylinder and crank-case halves are tied together, they might have some surfaces not perfectly matching which create edges, there is a way to machine the adherent areas to smoothen them down and create a good match. This is done by mounting one of the two halves of the crank-case together with the cylinder and machining the two matching surfaces aligning them as best possible around the area of the lateral transfer duct (or ducts) and the central transfer duct (called T duct). Same thing must be done mounting the other half of the crank case with the cylinder. We have now a smooth entrance of the mixture through the first part of the lateral transfer ducts and the T duct. T duct can also be machined in its internal surface that often has a sharp edge given by industrial machining. Rounding such edge optimizes mixture flow. To be sure that such job reaches full result we must also verify that the gasket which is positioned between the cylinder and the crank-case does not obstruct gas flow inside the lateral transfer ducts and the T duct. If the gasket is wider than the duct there is no negative effect on gas flow, but if the gasket is narrower than the duct, an edge is created and turbulence generated. To avoid this, cut the gasket to the right dimension with a paper cutting blade.
Another area to check is the contact line between the cylinder liner and the aluminium of the cylinder. Often in fact the mounting of the liner inside the cylinder, which is done by heating the cylinder (so it expands) and partially forcing the liner inside, may finally not be perfect and create some edges that need to be machined to obtain a smooth surface which helps gas flow. Finally mixture reaches transfer ports. These must be machined in the edges to avoid having them too sharp. Sharp edges in fact create turbulence of gas which leads to resistance to flow. The effect of turbulence is equivalent to a reduction of the port area, and the result is the same as having mixture passing through a narrower duct. Machining has the aim of rounding sharp edges of the transfer ports. This reduces considerably turbulence when mixture exits the transfer duct through the ports into the combustion chamber. There is though a limit to rounding edges of the ports. It is in fact true that an excessive rounding generates an imprecise opening and closing of the ports when the piston passes over them. This can lead to bad scavenging and lower performance of the engine. Rounding should be as limited as possible to avoid sharpness.