The key aspect of an engine cylinder, and more specifically of its liner, is that it should be perfectly circular. That is why, as time passes, it may need lapping, i.e. honing to restore optimal surface conditions.
Indeed, a cylinder with an oval liner has certain points with a lower piston-cylinder tolerance (therefore at risk of seizure) and others with a higher tolerance (reduced seal). Also, tolerance with respect to the piston in a conical cylinder varies from the top section to the bottom, resulting in differentiated levels of seal and, also in this case, difficulty in adjusting tolerances properly.
Another important aspect is the roughness of the cylinder surface, which is fundamental from the point of view of the seal and lubrication.
The lapping of a cylinder liner must undoubtedly be performed after engine failure, following a seizure, for example. It must be said that, on these occasions, the pistons usually end up being the most damaged, given that they are made of aluminium, leaving molten material on the cylinder liner which, on the other hand, being made of cast iron, is more resistant and hard to scratch. Sometimes, therefore, seizure merely results in a layer of aluminium on the cylinder liner, without damaging it. In this case, lapping simply serves to remove the aluminium from the cylinder.
In more serious cases, on the other hand, the cylinder is also scratched and lapping will be more invasive because it needs to remove more material from the liner.
Sometimes, as mentioned, lapping is performed to restore a cylindrical shape to the liner. Extensive use of an engine, with it repeatedly heating up, causes the liner to become worn and it loses its cylindrical shape. In particular, following many hours of use, a liner can become conical, widening in the upper part on the side of the head. This is due to the fact that the pressure generated by combustion tends to widen the ring which, therefore, presses more on the cylinder when it is closer to the top dead centre, accentuating wear on that area.
Lastly, wear also leads to the loss of a large part of the micro roughness on the inner wall of the cylinder, which is necessary for a layer of oil from the mixture to settle and remain on the liner to optimise lubrication.
Lapping a cylinder must be carried out using a lapping machine. It is a machine that has a rotating head made of two sanding stones (two pairs with two different grits) and two cleaning brushes (which must also come in two pairs, combined to the sanding stones). Professional models have adjustable rotation speeds and an on-off pedal that allows the operator to have both hands free. They are also equipped with an oil drop spout lubrication system, which maintains the rotating head - cylinder liner pairing always perfectly lubricated.
A bench-top lapping machine also has a fixed comparator gauge, which indicates how much material is being removed. Basically, it is reset at the start of the task and is then used to gauge the variation (increase) in the minimum diameter of the liner as work progresses. It must be said, however, that this is only an indicative value, given that variations are represented by one hundredth of a mm and said value also includes the wear on the stone.
There are also cheaper solutions, which can be used track-side. In truth, they basically involve just a polishing tip, similar to that of a bench-top lapping machine, which can be connected to a normal power drill, preferably with variable rotation speeds. Obviously, machining will be less precise, especially due to the impossibility of keeping the rotating head perfectly in line with the cylinder axis, but it is still a good track-side solution in case you need to lap a cylinder quickly.
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