THE FASTEST PLUG.
THE average motorist is perhaps inclined to take that important component, the sparking plug, rather for granted, but the sportsman, each cylinder of whose high-revving engine may easily need 20 sparks a second, does not take these things so lightly. All credit then to the plug manufacturers, who have developed their products from the fragile article which so infuriated an earlier generation of motorists, to the reliable component which is utilised in the cars of to-day.
Lodge Plugs Ltd., the oldest firm of Sparking plug manufacturers in Great Britain, was founded in 1904 by Alex and Brodie Lodge, sons of the famous scientist, Sir Oliver Lodge, and began operations in Birmingham. In 1913 the Company amalgamated with the Mascot Company of Rugby, and later moved to the specially built and equipped factory which they occupy at present. During the War, the entire resources of the factory were devoted to the manufacture of aero plugs, a line of development culminating in those used in the successful Schneider Trophy seaplanes in 1929 and 1931, which also captured the World Air Speed Record.
Our first question on beginning our tour of the factory was the reason for using Porcelain or stearite insulation on the C.3, the ordinary touring plug, and mica for the sports type. We were told in reply that the C.3 was a mica plug, like the sports pattern, the stearite being used merely for protecting the insulating layers. When the first sports plugs were produced, however, it was decided to use a mica covering, as the sporting motorist mentally associated a mica outside with speed. The porcelain actually has considerable advantages, as it is less troubled by condensation and is of course waterproof. Actually a porcelain insulated plug is still produced for very oily engines, notably the old T model Ford, so we went on to witness their manufacture.
The raw material in the form of powder is pressed into moulds, and further shaped to the required contour. The blanks are then stacked on fireclay trays, 60 at a time, and are carried on a continuous chain through a gas heated oven, 10 feet long, the hottest part being in the centre. They cool off gradually at the far end and emerge hard and dry. After this they have to pass through the vitrifying oven, a brick structure 230 feet long and ten feet high. They are carried through once more by an endless chain, the reduction from the motor to the sprocket being about 56,000 to 1, and the complete traverse takes two days. The maximum temperature is about 1,500° C. The central electrode is then inserted and cemented in position.
Mica insulation, as has been said, is used in most of the Lodge plugs. The raw material arrives in sheets about six inches square and about quarter of an inch thick. Apart from its excellent insulating qualities its most important property is the ease with which it flakes off into sheets as thin as 1 thousands of an inch. The splitting is done by girls who roughen a corner of the sheet on a piece of sand paper, then with a dexterity born of long experience apply the roughened edge to a knifeblade, and flake off amazingly thin layers. Any defect they spot at once, and the pile of rejected material which soon accumulates shows the high standard of perfection required.
Meanwhile in another department the centre electrodes have been made. Mild steel rod is used in plugs of moderate heat resisting cinalities, nickel or alloy points being pressed into a hollow in the lower end, the other end being threaded to receive the terminal nut, and also a copper ” radiator ” in the case of the ” hotter ” types. A solid copper rod is used in the racing types. The centre electrode is then sheathed in copper to increase its heat-conducti
vity, and wrapped with many layers of mica. A steel sleeve is then forced over the mica consolidating the whole, and forms the bearing surface for locating the centre assembly when the plug is built up.
The mica wrapping has now to be protected from injury either by using a series of mica washers as in the sports plugs, or a stearite insulator as in the touring C.3. Where mica is used, a special resilient washer prevents the lower washers from being crushed against the top surface of the sleeve, and a washer screwing down the centre electrode clamps the whole assembly in place. The outside edges of the mica washers are then polished to give them the finished appearance which is the pride of the enthusiast.
The centre is then ready for testing, and the top of the electrode and the metal sleeve are supported in insulated clips. A 12,000 volt current is then turned on, and a sheet of blue flame flickers across the insulation. In a defective centre the current chooses the easier path through the insulation and no flash appears. A pressure testing-machine tests the joint between mica and sleeve, which is subjected to a pressure of 120 lbs. for half a minute.
The plug-bodies, sleeves and gland nuts are made on automatic machines which are fed with hexagonal or round rods of the appropriate size. A battery of tools operate in turn and in a few seconds the body or other part is turned, bored and cut off to the right size, and then passes to the threading machines for tapping and finishing. The threads and other dimensions are then inspected and checked for pitch and size, after which the parts are ready for the rust proofing process. Sharrading, as the process is called, is carried out by heating the parts together with a third of their weight of zinc. The zinc is absorbed into the pores of the metal and proofs it against oxidisation, without
altering in the slightest the dimensions of the articles treated. The parts are now all complete except for the fitting of electrodes to the plug body. Discs are punched out of steel sheet, and these in turn have pieces punched out of them, lea i
%%rig a washershaped disc with one or three interior projections, which form the points. The discs are then pressed on to a cone-shaped mandrel which bends up the points to a small extent. The disc is dropped into a recess already machined in the bottom of the plug body and the side wall pressed over it, as is plainly shown in the sectional drawing of the H.1 plug. Disc electrodes, which are used in all Lodge car plugs, are much more easily adjusted than the wire type, which project at right angles to the body, since for gap setting the points only require to be bent up or down.
All is now ready for the final viewing and checking of threads, after which the plug is packed and goes to join the 500,000 others which are kept in stock. 45,000 is the weekly output and shipments are constantly being made to all parts of the world. Plug design, like most branches of automobile engineering is a matter of compromise. Touring engines are cool running and oily, while racing engines are as a rule hot and dry. The sports car falls between these two categories, and so a suitable plug requires to deal with both oil and heat. Three conditions determine the use to which a plug shall be put. They are 1. Thickness and material of the elec
2. Length of insulation betweenCentre electrode and sleeve.
3. Gas space in the bottom of the plug. Thus in a touring plug, such as theC.3, thinelectrodes of nickel or alloyburn off surplus oil, a long centre insulator keeps reasonably clear of it, and a large gas space keeps the interior hot and further resists the oiling effect. The B.R. 39
at the other end of the scale has a thick copper electrode, a very short length of insulating and a tiny gas-space, so that it
does not become unduly heated up under the most strenuous conditions, but of course if the engine runs light and oil gets about the combustion chamber, the plug is almost certain to oil up. Incidentally a groove turned in the bottom of all the sports plugs has the effect of draining away oil which might chance to alight on the end, instead of letting it drip on to the points.
For sports car use the H.D. and the H.45 are recommended, the former for ordinary sports cars, and the latter for very high compression or supercharged vehicles, the 14 mm. equivalents being the H.D.14 and the 11.53. As has been indicated the ” softer” of these has an alloy electrode of stout proportions, while copper is used in the H.45, and the interior insulator gets shorter on the hotter plug, but is stepped slightly to increase the length of leakage path. The points are protected from oil by being fitted up inside the barrel. The H.50, a pepperbox type, hot enough for most car purposes, comes next, after which the types become non-detachable, with increasingly thick electrodes and the other properties which have been stated as appertaining to racing plugs, ending up with the B.R.39, which we should imagine would stand snore heat than has yet been reached in an internal combustion engine. In conclusion we would add that these notes do not pretend to do more than to point out the principles which determine the heat or oil resisting qualities of plugs. Anyone wishing to use them for a specific purpose, trials, racing, or touring, should get in touch with
h Lodge Plugs, Ltd,. of Rugby, who can supply all in f or mation. and recom men d suitable models.
The fir in ‘ $ vast experience in assisting in the breaking of world’s records is at their disposal.
Rumblings, June 1932
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