Tying Down Some Bore Wear Myths
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BASED primarily on experience gained with private-car engines, Mr. W. A. Robotham's paper entitled "Some Problems of Cylinderbore Wear," which he recently read before the Institution of Automobile Engineers, is exceedingly interesting.
Mr. Robotham explodes many preconceived ideas as to the causes and cures of bore wear and, although his contentions may be challenged in some quarters, they are based on actual experience over many years. In dealing with the periods between rebores, the author suggests that an engine, no matter how adverse t'..e operating conditions, should not need reboring under 50.000 miles.
The significance of this target figure is emphasized by the fact that the useful life of an engine between rebores varies between 15,000 and 75,000 miles. Figures are given showing the average wear in an average car engine, after 48.461 miles, to be 0.000167 in. per 1,000 miles, and, in a bad case, 0.000695 in. per 1,000 miles on a mileage of 20,183.
High Average Speeds
Just how such extremes in bore wear Come about is explained and it can be deduced that the vehicle operated for long stretches at a high average speed shows much less bore wear than does a machine running for the most part in London traffic. Extremes in these cases can be 100,000 miles, and as low as 15,000 miles, between rebores.
Nondetonating conditions, the author indicates, are conducive to minimum wear, Whilst repeated fullthrottle acceleratien probably is responsible for undue wear. Mr. Robotham goes on to refer to statements which are frequently made to the effect that bore wear can be reduced by paying attention to the distribution of the lubricant, piston and piston ring design, and the means adopted for oil control, as opposed to the use of special materials in cylinder blocks.
Check on American Vehicles
' Bore wear has been stated to have been reduced in this way on American vehicles, but the author checked this assertion, and the results did not substantiate any such claim of American superiority in this direction.
One of the most common of beliefs is that if an engine be copiously supplied with oil, cylinder-bore wear will be minimized. An adequate supply of lubricant is obviously necessary if any engine is to function satisfactorily, says Mr. Robotham. He adds, however, that adequate lubrication can hardly be claimed as a modern discovery.
He shows, in graph form, how oil consumption increases with engine speed and, if the unit pressure of the standard rings be increased to modify this, the consumption under low-speed conditions becomes microscopic, and the resultant boundary lubrication conditions accelerate cylinder-bore wear.
The author, in referring to the small degree of bore wear in engines produced in the early 'twenties, which, of course, were essentially of the lowspeed type, says that this seems to be confirmatory evidence that engines with a small speed range show good bore life.
One would scarcely associate the advent of front-wheel brakes with accelerated cylinder-bore wear, but Mr. Robotham introduces this point from the aspect that drivers take a higher output from their engines with the increased facility to stop the vehicle.
That the rate of bore wear decreases after it has reached from 0.010 in. to 0.014 in., when an undesirable quantity of oil is undoubtedly passing the pistons, is an interesting point. Regarding this the author says that the rate of wear falls to 30 per cent, of the figure for the first 20,000 miles.
Unlimited Lubrication
After making experiments in the direction of feeding an unlimited quantity of oil to the cylinder walls, Mr. Robotham has been forced to the conclusion that "there is no magic • in it." The bearing of oil viscosity on bore wear provides much material for discussion.
The author says that, in his experience, a thin, oil, despite the fact that it gets to the top ring quickly, does not give a marked reduction in wear. The use of thin oil, Mr. Robotham points out, means that the severity of ring control has to be increased to avoid high oil consumption at maximum revolutions, and this may well counteract the improved lubrication at starting.
Whilst it is always desirable to keep the engine-oil clean, either by filtration or frequent crankcase drainage, the author says that he has detected tittle connection between bore durability and frequency of oil changes. He also says that, basically, he feels that the vehicle user should not be asked to mix anything with his fuel.
Unequal bore wear in the cylinders of an engine may, he say% be caused by increased detonation in worn bores, which arises from variation in ignition timing or mixture distribution.
Dealing with piston and piston-ring design. Mr. Robotham says that records show it is possible to aggravate cylinder-bore wear by fitting a piston the skirt of which conforms so closely with the cylinder that it precludes an adequate quantity of oil from reaching the ring band.
A definite contribution towards the improvement of piston life in recent years has been the introduction of tinplating. The tin coating lasts much longer than one would expect, the author having observed signs of the coating after 10,000 miles of running.
High-pressure Rings
Tests have indicated that rings of high unit pressure are not necessary to obtain good oil consumption, and can be dangerous if there be even a momentary shortage of lubricant. The author recommends the use of piston rings of the lightest pressure consistent with the securing of the desired economy. A good combination, so far as types are concerned, is one gas ring, one L-secn ring, and one slotted scraper ring. When the slot clearance in the piston exceeds 0.003 in. oil consumption may be expected to increase.
Cylinder-block insert materials having 16 per cent, chromium or more, have been shown to reduce wear to about one-seventh of the normal figure, and chromium plating gives similar favourable results.
Good Results from Plating
At the present moment, he says, the best liner materials are expensive and, therefore, must not be used wastefully. If the required life can be obtained by using a liner on only the upper half of the bore, there seems to be little justification for full-length insertions. Chromium plating on a cylinder surface seems to have as good a life as any known commercially obtainable material. Full-length plating of appreciable thickness has given consistently good results, he says. In some cases trouble was experienced, initially, in maintaining the oil film, but this was overcome by carrying out the plating in such a manner as to produce what is sometimes known as "porous chrome."
It is a curious fact, says the author, that reducing the amount of wear by chromium plating the upper portion of the cylinder bore, also appears to reduce the rate of wear of the remainder of the bore.
The conclusions Mr. Robotham has arrived at, following tests of an engine in a cold chamber in order, if possible, to accelerate bore wear, are that, given a reasonably good system of lubrication, and oil of the correct viscosity, there is no necessity to take any abnormal precautions to warm up an engine slowly.