Most engines have a knock of some sort which annoys the driver, which might vary from a barely perceptible click to a blow that may be heard by persons standing on the sidewalk. A great many of the knocks do not do any harm; that is, they are not causing any damage to the engine parts; but there are some knocks which require immediate attention to prevent the demolition of the engine or seriously damaging it. Some of these are loose connecting-rod bearing; cylinder loose on its base; lack of lubrication and consequent overheating; and a broken revolving or reciprocating member, which might tear things up if allowed to run. The spark advanced too far puts an excessive strain on the crank shaft, and a short circuit in the ignition does the same.
On the other hand there is no immediate danger if there is a piston slap, though pretty noisy; nor is there when the oil pump gives a thump or loud click at the valve seating. But on general principles when there is a knock its cause should be ascertained at once and expert advice be obtained as to whether it is dangerous. If so, it must be fixed before running further. If it is only a minor knock the repair can be left to a more convenient time.
So many of the knocks are only a matter of annoyance; that is, they do no more harm than to cause the driver unnecessary nerve jars and to spoil the pleasure of riding. But he will bother the various garage men and everybody else he can get to listen and then will not accept the assurance that the knock is not doing the car any harm.
As a matter of fact there are knocks peculiar to certain makes of engines that the manufacturers have not seen fit to eliminate because they are not harmful. Among these is the so-called “gas knock.” Now the writers do not know what a “gas knock” is, and doubt if anybody else does. But it is a common term for the kind of a knock which cannot be located anywhere and it is laid to some action of the gas.
Sometimes the knocks are hard to locate. One of the school cars needed a new cam-shaft gear to replace a worn one which produced a knock. It was put on, but when the engine was started it was found that there was a worse knock than before. It took a long time to find that it was the new gear. It fitted very tight on the shaft and had to be forced on. Without being noticeable the forcing had cracked the hub, the crack extending through the rim, so that two of the teeth were out of true and had spread enough to pound when that portion of the gear came into mesh.
Another knock difficult to locate is caused by sticking exhaust valves. One of the valve stems has been oiled, say, and this has been allowed to burn to a nice sticky paste. Now, when the push rod opens the valve and starts down the valve does not follow closely. The spring brings it down, of course, but more slowly than it should, because of the sticky stem, and when the stem and push rod meet there is a thump.
It is ordinarily said that a lean mixture or an over advanced spark will cause knocking. As a matter of fact they do not do anything of the kind; but it is true that these things bring to our notice other faults, such as a worn piston or cylinder wall and consequent side slap, or they magnify some other worn part so that it becomes audible. As a matter of fact it is possible to advance the spark so far that it will try to reverse the engine and not have a particle of knock. And it is possible to have a mixture so lean that it will hardly run the engine yet have no knock; but if there is something else wrong it will bring that out.
Some of the causes of knocking discovered at the school have been collated and are given for the instruction of the novice:
Mechanical Looseness, Due to Improper Adjustment, or Wear:
Loose connecting-rod bearing, crank-pin end.