THE SPARK PLUG

New Zealand Times, Volume LII, Issue 12116, 18 April 1925, Page 14

THE SPARK PLUG

AN IMPORTANT FACTOR,

Few people realise the important factors which are involved in the manufacture of a successful spark plug. Every piece of material entered into the production of plugs must pass through several different inspections before going into the product. The steel shells are all examined for cracks, imperfections; and threads are checked with the proper gauges before passing to the assembly department. The most important item in the construction of this plup is the mica, and it has been found that no mica has yet produced the service and results os has been obtained by the Indian Ruby mica, which is the most expensive mica for this class of work on the market to-day. Almost equally important as the mica are the firing pins or electrode point of the plug. In the old days it was rather difficult to get a material which would withstand the intense heat of a combustion chamber plus the action of a good high tension magneto, and only within the past few years have alloys been developed which are extremely fine and durable for this purpose. MANUFACTURING THE PLUG.

The process of manufacture of plugs is approximately as follows The tough mica is punched in long narrow strips, and is then split by highlytrained operators until each piece is from .OOlin to .003 in thick. The mica then passes to the winders and is wound on a tapered mandril until a cone-shaped sleeve is produced of the desired thickness to enter the tapered hole in the bushing of the plug. The mandril upon which this sleeve is wound is another important factor, and a variation of only a few degrees in this taper would make quite a difference in the finished plug. Too next operation is that of driving in the tapered pin forming the centre electrode. which operation is done with a heavy trip hammer of free, determined force to drive the centre electrode tightly into the bushing. These pins ere so tight that it is often found, when trying to drive one out, that the pin will bend before it becomes dislodged in the hushing. After this operation, the electrodes are tested with a high tension current, producing a spark .approximately Jin long, and this spark must jump over a surface of the mica, showing that the insulation is good—otherwise the assembly is rejected and scrapped. The elettrodes are then passed on to several minor operations, such as dialling a hole foi* the firing pin, removing burrs, and grinding a smooth, tapered finish on that part of tho mica which extends into the shell.

Next, the firing pins are driven into the end of the electrode stem and spun in with a hollow punch to ensure th'eir tightness. After this operation, the electrodes are again tested on a high tension spark to ensure no damage has been done to the insulation of mica. The electrodes are then passed to the assemblers, where they are inserted in the proper shell assemblies and with a soft copper gaskefc helow the shell and bushing are screwed together by machines to ensure no leakage at this Joint. It is a curious thing that the general, knowledge of motorists, embracing as it does all details of pistons, cylinders, valves, overhead menhanism, and the like, is exceedingly poor as regards that most important factor, the sparking plug. Many otherwise discriminating motorists appear to he perfectly satisfied with the sparking plug. provided that it does not oause misfiring, and, should this occur, they often seek to remedy it by washing out the plug with petrol and brushing the points.

It is not uncommon for a motorist to condemn certain makes of plug as useless because he has tried one and found it unsatisfactory in his engine. It is quite possible that it was altogether unsatisfactory because it was the wrong type. TYPES OF PLUGS. There is almost as much difference between various types of plugs of the same make as there is between lead and nickel-steel. Sparking plugs may well be compared to electric lamps—a 100volt lamp will give excellent service on a 100-volt circuit, but if it be used on a 220-volt circuit it will fuse almost immediately. In the same way a touring plug will stand in a touring engine, but will at once give trouble in a racing or high-compression engine. The converse is also true.

It is not difficult to understand why this is so. Sparking plugs can he manufactured to stand (1) a lot of oil and very little heat or (2) a lot of heat and very little oil. These are the two extremes, and plugs are, of course, made in all varieties, ranging between these extremes; but q plug cannot be made to stand both oil and heat. The plugs, therefore, will resist either heat or oil, or a reasonable combination of the two. It must be carefuljy selected, then, for the particular engine in which it is to be used. A low-compression touring engine is not really hot (as regards flame-heat), but it is as a rule oily, and therefore it requires a plug which will stand oil; a high-compression, fast-run-ning engine, on the other hand, has a great flame-heat, and its plug must be capable of resisting this heat and working satisfactorily in it. The qualities of a plug are decided, in the main, by the size of its gas cavity and electroids. Broadly speaking, a small centre electrode and a large gas cavity give a plug which .will ■ stand oil, whilst a large electrode and a small cavity will stand heat, hut not oil. The gas cavity is, of course, the space between the centre electrode and the body of the plug or insulating materials. Motorists desirous of obtaining satisfaction as regards plugs should be careful in purchasing, and be sure that they are obtaining plugs suitable for the engine in their car or motor-cycle.