The Running-in of Engines: Choice of Cylinder Bore Finish

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Abstract
The process of mating of piston rings and cylinder bores in newly assembled Diesel and petrol engines is described, and it is shown that an optimum surface finish exists which leads to rapid running-in—or break-in as it is sometimes termed—when either straight mineral or additive type engine oils are used. In particular it is shown that the running-in of engines fitted with either cast-iron or chromium-plated piston rings may be of short duration when additive type engine lubricants are used in conjunction with a suitable bore finish. Rig and engine tests are described which demonstrate the influence of surface finish on both the wear and the consumption of oil which occur during the running-in process. It is suggested that there are two components of surface finish which affect the running-in process. One of these components, the surface undulations of short wavelength (surface roughness) influences the rate of wear during break-in, whilst the surface undulations of longer wavelength (surface waviness) determine the amount of wear which must occur before the running-in process is complete. Whatever surface finish is chosen for the cylinder bore, to achieve a break-in which is as rapid as possible it is desirable to eliminate those errors in the machining process which give rise to waviness.

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