Curious Facts About
Page 58
Page 59
If you've noticed an error in this article please click here to report it so we can fix it.
CYLINDER WEAR
Important Observations upon a Topical Question. Interesting Explanations as to Causes and Effects. Position of Valves and Sparking Plugs
as Dominant Factors
By A Repair Specialist
WEEN anyone asks what is the cause of cylinder wear it is surprising the number of totally different reasons that are given. One will say that cylinder wear can be traced to a certain type of piston, another will say lack of lubricant or diluted lubricant, another that excessive wear is the result of using alloy pistons, or, perhaps, the blame is attributed to the rings, or to high compression.
The writer is in a position to see and study these things, because he is in charge of a plant put down for the reboring and sleeving of worn cylinders of all makes and sizes; the plant is run by One of the first commercial-vehicle concerns to specialize in this particular branch of the trade.
Inequality of Wear.
Coming into contact with so many different types of cylinder block, one thing quickly became apparent -when reboring oversize, before sleeving was so popular. This was that, although the dial gauge might only show 10 or 12 thousandths wear in diameter, nothing less than 30 thousandths oversize could be safely depended on to clean up the worn bore and take out all trace of wear, because the wear was not equal all around the bore. Thus, ont of the 12 thousandths mentioned, there would be 4 at one side and 8 at the other, sometimes as much as 2 and 10 at opposite points, making up the total 12. When this occurs over and over again one naturally begins to ask why this should be so.
Pursuing one's own line of thought, great difficulties are encountered when trying to attach the cause of the wear to any one particular component. First, we have the pistons, cast-iron, alloys too numerous to mention, pistons with pressed-steel skirts and alloy crowns, c30 and many other types, simple and complicated,. some with the gudgeon-pin situated near the top of the skirt, some near the middle and others very low down.
Taking an alloy piston with a long, plain skirt, one might reasonably assume that being made of fairly soft metal it could pick up particles of carbon, metal and other impurities in suspension in the lubricating oil, such as road grit, and, acting as a lap, grind the bore away quicker than a cast-iron piston would do.
Or, again, that with the bigger clearances allowed for alloy pistons, when starting from cold, more neat petrol would pass the rings and skirt, washing the cylinder walls dry of lubricant more quickly and for a longer period of time, resulting in heavy dilution of the oil in the Sump. Yet it is a fact that the lower portion of the worst worn bore rarely shows more than 1 or 2 thousandths wear, no matter of what metal the piston is made, although the full skirt length passes this point at every stroke.
Again, we are told that the thrust
on the piston on the compression stroke forces it to one side of the bore—the right-hand side looking from radiator— whilst the explosion. kicks it over to the other side. In an attempt to reduce this we get inclined bores and offset crankshafts.
If we place two cylinder blocks side by side, one of British manufacture with valves on the near side, and the other of American or Continental make, with its valves an our off side, although both engines revolve in the same direction, the point of maximum wear will be found on opposite sides of the respective bores. Each block will show more wear on that side of the bore opposite to the valves and, incidentally, in most eases, opposite to the position of the sparking plug.
Fore and Aft Troubles.
If we now take an overhead-valveengine block, it will be found that the wear is nearly equal i41 around the bore, but again the point showing maximum wear is on the opposite side of the bore to the plug. If a worn cylinder bore has its point of maximum wear either fore or aft, instead of athwart as usually is the ease, this can be traced definitely to pistons out of line in the. bores, bent or twisted con.necting rods, a crankshaft not parallel with the cylinder base, or sometimes to very loose big-ends, or to a slender crankshaft that is subject to a good deal of whip.
Coming now to piston rings. All the wear in a bore comes within the limits of the ring track. It has its maximum value at the exact spot where the top ring finishes itstravel, falls away rapidly for the first inch down the bore and then gradually to the bottom end of the ring track ; beyond this, to the bottom of the bore, the cylinder is nearly standard size. The writer has checked a worn cylinder block showing 31 thousandths wear at the top of the ring track, yet the last 4 ins, of the bore below the ring track showed but 2 thousandths wear.
This again raises a difficult question. At the point of maximum wear the piston, and with it, of course, the rings, is theoretically at a standstill at the end of its stroke, but at the part of the bore where the piston speed reaches its maximum value little wear is shown. Perhaps we can account for this by taking compression and firing into consideration.
However, if we study the modern trend of design in cylinder heads, especially Ricardo-type heads, it will be noticed that with the combustion chamber confined to a space over the valves there is only a layer of charge the thickness of the gasket over the larger part, say two-thirds, of the piston ; this will be fired last of all, after the piston has started on the downward stroke, and thus will have left the point where the maximum wear occurs.
There still remains a theory that, on the compression stroke, the space in the groove behind the top ring becomes charged with gas leaking through the gap, and this, with the help of the hot carbon particles and oil already in the groove, distends the ring ; at the top of the stroke, the action of coming up against the cylinder head with what we might call "a snap," distends the ring still farther, the entire charge being fired at or about this point according to the setting of the ignition.
As the top ring wears thie state of affairs begins to occur in the second ring groove as well, thus we get more oil and carbon behind the top ring resulting in accelerated wear both on the ring and cylinder walls at the top of the stroke. In any case it provides a perfectly sound reason for the top ring wearing much more rapidly than the rest.
Another point that has a bearing on this subject is the clearance around, and the depth of, that part of the piston above the top of the ring groove. If this clearance be on a generous scale, as it generally is with alloy pistons, when it may amount to 25 or 30 thousandths on a 4i-in. to 5-in, piston, or if the distance from top ring groove to the crown be short, as is the case with many cast-iron pistons, the compressed charge has an easy path to the top piston-ring gap and from thence to the space behind the ring.
Another point is that in engines with piston crowns hollowed out to obtain a hemispherical combustion chamber, generally associated with overhead valves, the wear is nearly equal all around the bore, the position of the plug being the deciding factor as to the point where the maximum is to be found.
This wearing of the bore seems to be definitely the result of ring travel and nothing else, but it is influenced by the design of the piston, position of valves and plug and, last but not least, by the material or quality of the metal of the bore.
We had the opportunity, not long ago, for checking the cylinder of a directinjection oil engine with the nozzle situated directly in the centre of the bead and noted that the little wear shown was exactly equal around the bore and confined to the top inch of ring track.