Enthusiast's Directory: No. 2 - Sparking Plugs


Although petrol-injection may one day render the carburetter obsolete, the sparking plug looks like staying with us until the bitter end—for the piston engine—when the turbo-jet takes over. In the beginning hot-tubes were used for ignition purposes, superseded by low-tension ignition in which the make-and-break was situated within the cylinders, for in those far-away days a camshaft and push-rods actuated a make-and-break which was part of the plug. The difficulties of keeping such cumbersome sparking plugs cool and of rendering them gas-tight can well be imagined! A notable user of l.t. ignition today is Dr. Ewen's 1908 G.P. Itala.

As the high-tension magneto was perfected l.t. ignition died out and the function of the sparking ping became that of a spark gap, the make-and-break being transferred to the magneto.

Up to 1914 all manner of odd plugs were put on the market, including one in which the spark could be observed through a glass window, plugs with china insulation and so on. The experiments of K. Lee Guinness on racing-car engines led to the commercialisation of the famous K.L.G. sparking plug, fortunately just prior to the 1914-18 war, in which these plugs, which in those days had scores of mica discs forming the insulator, gave yeoman service in the air.

In the years following the war plug design began to settle down and for a long time the 18-mm. (22-mm. in America) single or multi-electrode plug, often detachable for purposes of cleaning, became universal. Later, as cylinder-head research progressed, the 14-mm. plug and the 10-mm. plug were introduced to enable the uncooled portion of the combustion chamber to be kept to as small an area as possible.

In recent times Lodge racing plugs have made notable headway, one grade being suitable both for warming-up and racing most engines, whereas at one time "soft" (i.e., hot) plugs were always changed for "hard" (i.e., cool) plugs just before the start of a race. If a cylinder cut-out during the race these "hard" plugs, being too cool to burn oil off their electrodes, invariably packed-up for good, necessitating a call at the pits. Lodge, with their universal racing grades, secured the patronage of Alfa-Romeo team—a signal honour.

Another development is that of using very wide plug gaps in conjunction with a high-voltage coil to enable the spark to bridge so wide a gulf, in the interests of economy. This was pioneered on the pre-war Vauxhall Ten, but is really an ignition rather than a sparking plug development, as the carburetter jet sizes are cut down to decrease petrol consumption, the point of the wide plug gap being to produce a "long" spark able to ignite the resultant weak mixture. Yet it is significant how many people open up the plug gaps and expect improved economy without altering carburetter settings!

To determine which plug suits a given engine, the charts issued by the plug manufacturers and available free on application, should be consulted. In the case of modified or special engines the Heat-Range Chart on page 443, which has been compiled in conjunction with the plug makers, will form a useful guide; it is also a guide when the required grade is known, but the particular make is not available.

Most makers recommend a new set of plugs after about 10,000 miles. It should he appreciated that a plug may spark satisfactorily under atmospheric pressure, but develop h.t. leaks under engine compression. Testing machines are available with which to detect such faults.

The car handbook should be consulted as to correct plug and contact breaker gaps, but as a general rule Champion recommend a e.b. setting of .010-.012 in. (coil) and a plug gap of .022 in. with coil, .018 in. with magneto ignition, and Bosch a plug gap of 0.5-mm. for magneto. 0.7-mm, for coil. Remember that too small a c.b. overloads the condenser and burns the points. Too wide a gap means a weak spark. Insulator temperatures up to 1400 deg. F. are common, increasing to 2,000 deg. F. when pre-ignition takes place; 80 per cent. of the heat is conveyed away via the plug washer and for this reason a plain copper washer, annealed if possible, is preferable to a C. & A. washer. When fitting a plug do not use too great a tightening pressure. Champion recommend 40-45 foot-lb. torque for 18-mm., 32-38 foot-lb. for 14-mm. and 18-20 foot-lb. for 10-mm. plugs, Bosch 5-5.5 metre-kilograms for 18-mm., 4 m/k. for 14-mm., 3 m/k. for I2-mm. and 1.5 m/k. for 10-mm. plugs. With non-mica plugs an ashy white insulator indicates over-heating, dull velvet and black under-heating and a brownish shade (light carbon and oil film and greyish lead deposit permissible) correct plug temperature and mixture.


Developed from the famuous A.C.-Sphinx plug, the present range of A.C. sparking plugs is intended for normal cars and sports models, but not for purely sporting or racing work. The insulators are aircraft type. These plugs are made by a division of General Motors Ltd.

Makers.—A.C.-Sphinx. Spark Plug Company. Dunstable. Bedfordshire (Dunstable 1166).


Bosch electrics gained a very high reputation prior to the war and a full range of Bosch plugs is again available. They are single or double-electrode plugs with Pyranit insulators, costing 5s. 6d. each. The type designations indicate the plug characteristics. The first letter indicates the size (M = 18-mm., W = 14-mmn., U = 10-mm., X = 12-mm., Z --English thread). The following number designates the heat value, the next letter the type of insulator (A = Pyranit 1 with single green ring, T = Pyranit 2 with two green rings) and the last numeral the general type, i.e., thread, reach, wrench size and type of electrode. A swan-neck side electrode is N-type, double electrode W-type, a single flat-side electrode X-type, D = a separable plug as distinct front a non-detachable plug.

Besides plugs weatherproof covers, terminals, gauges in neat cases and plug cleaners are listed. There is also a rather Edwardian "Pocket Encyclopedia" (No. 1 in the Bosch series) giving a vast amount of data under alphabetically arranged headings; it is in English and costs 1s. A plug repair depot is maintained at Warple Way, W.3.

Sales Manager : S. Bright.

Agents : Bosch. Ltd.. 43, South Audley Street, London, W.1. (Grosvenor 3814/5).


Champion-plugs originally came from America and their increasing use in sports-type cars like the Bugatti and their subsequent racing successes, established them amongst the leading makes as a sparking-plug particularly suited to high-performance engines.

Today they are used by Ferrari and other outstanding racing teams, are fitted as standard by many British manufacturers and were used by Gordon Wilkins and J. Lowrey when these drivers won the premier honours in the News Chronicle Petrol Econonty Contest with Javelin and Morgan Plus Four cars.

Champion car-plugs are of single electrode type, with Champion Ceramic insulation, Sillment compressed dry power compression seal and patented special alloy electrodes which have a catalytic agent to obviate bridging of the spark gap, or oxidisation or carbon formation on the electrode wire.

Champion car plugs cost 5s. each, with the exception of the NA-10, long-reach 14-nmm., which costs 6s.

Champion accessories include various types of terminals, including those for racing plugs at 1s. 6d. per dozen, a 2s. gap-gauge and adjuster embodying feelers for .018, .020, 025, .028, .030, .033, .035 and .040 in., 1s. 6d.waterproof plug-covers, and a variety of Service Units for cleaning and testing the insulation efficiency, gas tightness and spark intensity of old and new plugs, these Units being intended for service station use and priced at £15 15s.

Three booklets are issued by Champion which should he read by all enthusiasts who are interested in this aspect of i.e. engine efficiency --the catalogue, which lists the Champion range and includes recommendation and heat-range charts, "Facts About Sparking Plugs and Engines" which covers this subject in elementary language, and "Sparking Plug Service Information," which not only comprehensively covers the plug itself, but also tells what parts the ignition system components, such as the condenser, etc play in ignition efficiency, and what causes pre-ignition, detonation, corona, etc. These books are available on mentioning Motor Sport.

Competitions Manager: H. W. Irving.

Manufacturers: Champion Sparking Plug Co., Ltd., Feltham, Middlesex (Hounslow 4494).


It is just forty years since the first K.L.G. plugs were made. Their introduction was due to the fact that the late Mr. Kenelm Lee Guinness, an enthusiastic amateur driver, found that the sparking plug of 1912 was the limiting factor of really high engine speeds and so he evolved one for his own use which, since those early days, has borne his initials. This plug proved so successful that his racing friends throughout the world asked him to supply them. So production on a very small scale was started in the cellars of an old house on the Portsmouth Rood, which had once been a hostelry known as the "Bald Face Stag."

As early as 1913, when the first Royal Aircraft Establishment test of British aero engines took place, K.L.G. plugs were used exclusively. On the outbreak of the first World War, as was to be expected, K.L.G. plugs found their way into early Service aeroplanes. They proved to be superior to all others, and Mr. K. Lee Guinness was asked to resign from the Navy, in which he was serving, and to concentrate upon the production of K.L.G. plugs on a large scale for the R.F.C. and R.N.A.S. Production was increased steadily right through the war period until, at the conclusion of hostilities, K.L.G. plugs were famous among flying men all the world over and in place of the modest building where the first plugs had been produced, there existed a modern well-equipped factory employing upwards of 1,500 people. In the course of the next fifteen years this was increased to eight times the size and today it is ten times as large as it was in 1919.

The association of K.L.G. plugs with the House of Smith has lasted for nearly thirty years, for just before the conclusion of the 1914 war, arrangements were then being made at the behest of the Air Ministry, for the produotion of K.L.G. plugs under licence by S. Smith & Sons (M. A.) Limited. This led Sir Allan Gordon-Smith to consider the desirability of adding the K.L.G. sparking plug to the complementary range of motor accessories which the firm were manufacturing, but Mr. K. Lee Guinness felt disinclined to sell and an arrangement was made on July 2nd, 1919, whereby S. Smith & Sons (M.A.) Limited acquired the world's selling rights. It was a natural corollary that eight years later the control of the business passed into the company's hands, Mr. K. Lee Guinness retaining a position as consultant.

There can be few businesses which have had more interesting associations than that of K.L.G. plugs. They were used from 1919 until the outbreak of this last war for many aeronautical and motoring achievements of note in Great Britain and abroad. In addition the company was at one time also responsible for the manufacture of special engines, and, in fact, complete racing cars such as, for instance, the late Sir Malcolm Campbell's "Blue Bird " and the late Sir Henry Segrave's "Golden Arrow."

In 1935 K.L.G. laid down plant for the production of Corundite insulators. This step was taken as it was evident that with the introduction of leaded fuels, the ultimate death knell of mica as a plug insulator would be rung.

The completion of the specialised equipment and development work on Corundite proceeded through 1935 and 1936 and the very first K.L.G. aviation plug having a Corundite insulation was designed on February 9th, 1937.

It was opportune that initial deliveries to the R.A.F. of plugs designed for the higher-powered Merlins and Vultures commenced a few days before war broke out, because Fighter Command removed the boost-limitation on the Merlin engine and there was an immediate demand for the higher-duty plug.

Large-scale production was achieved and maintained throughout the Battle of Britain and in 1944, when the company was told to concentrate all its resources upon producing every single Merlin plug possible, an output of well over a quarter of a million was reached in June. Despite flying bombs, throughout July and August record outputs were achieved and maintained throughout the autumn.

One can hardly quote type by type, but when the Typhoons were busting Rommel's tanks and railway communications, screaming demands were made by the Tactical Air Force for increased supplies of plugs for Sabre engines. K.L.G. quadrupled the output of these plugs within four weeks (at that time they were virtually the sole producers of Sabre plugs). The year 1944 produced an 80 per cent. increase on 1943; it is calculated that if the millions of K.L.G. aircraft plugs which were supplied during the war were laid end to end, they would cover 351.8 miles and the total weight of steel used in their production was in the neighbourhood of 2,201.7 tons.

Most of our readers will know of the high compliment which was paid to K.L.G. by the late President Roosevelt when the Corundite plug was the design chosen exclusively for reproduction in the United States by America's leading aviation plug makers. In addition to the aircraft side, thousands of K.L.G. Corundite plugs were developed and produced for the Navy's coastal forces and the Army's tanks.

Since the war the output of K.L.G. plugs has been further increased, and an extensive development programme has been undertaken further to improve the product.

The K.L.G. Engine Test House has been built and equipped, and already improvements have been made in design. For instance, in the case of the popular F50 plug, longer life is ensured, plus better idling. Also in the Test House the all-important plug-rating is carried out in an engine specially adapted for this purpose. Comparison charts can therefore be built up, and exact knowledge obtained of the heat and oil range of different plugs. Bearing in mind that the largest single cause of plug trouble is fitting a plug of the wrong heat-value for the particular engine, the K.L.G. engineers make full use of the Test House facilities.

Together with improvements in the design of the ordinary range of plugs, a completely new ceramic has been developed by K.L.G. Known as S749, this ceramic is being used as the insulator for the new K.L.G. ceramic racing plugs. The 1952 racing season was the first in which these new plugs have been used, and already it has been amply demonstrated by Hawthorn and others that they are fully up to their job.

K.L.G. are proud of the fact that the price of their "popular" sparking plugs is still 5s. This tells its own story of the vast improvements in methods of manufacture that have taken place, including a surprising degree of standardisation. Despite the fact that output has never been so high and that K.L.G. sparking plugs are better than they have ever been, research and development do not stop.

Competition Manager: R. G. Mundy.

Makers: K.L.G. Sparking Plugs, Ltd.. Cricklewood Works, London, N.W.2 (Glad. 3333)


As far back as 1908, Mr. Bernard Hopps, the present chairman and technical adviser of Lodge Plugs, Ltd., founded a company to manufacture sparking plugs. This company was amalgamated in 1913 with a company formed by the Lodge Brothers (sons of Sir Oliver Lodge) in 1904. Manufacture in 1913 was carried out in Birmingham and Rugby. In 1916 the company was moved to premises newly built in Rugby, where it is still based, the premises having, of course, been considerably enlarged since then.

The design of the Lodge mica racing plug was developed in the late 1920s and considerable success was achieved with these. Mr. Bernard Hopps, however, was convinced that it was possible, with sintered aluminium oxide, to produce an insulator that would supersede mica, which at best was expensive and difficult to work. In 1935, after very considerable research and experimental work in the company's laboratories, Lodge developed a ceramic mtaterial now well known as "Sintox." Its characteristic pink colour derives from the oxides which are combined with the sintered aluminium oxide. These are blended with other materials in such proportions that the resultant "Sintox" is remarkably hard, physically strong and has a thermal conductivity twenty times that of porcelain.

During the last war the company, under the Essential Works Order, concentrated on plugs for British, United States and Allied aircraft to the extent of 75 per cent. of its output, the remaining 25 per cent. devoted principally to special plugs for tanks and naval craft. By 1946, following exhaustive tests, a range of Lodge "Sintox" insulated racing plugs was introduced and this has been added to, as necessary, to cope with the latest developments in racing-car engines. The present range covers every need, as shown by results in Grand Prix and other International races - 23 "firsts" in 1949. 33 in 1950 and 24 in 1951 were gained by drivers using Lodge plugs.

The qualities of the Lodge "Sintox" insulator are matched by the care taken in the physical design and manufacture of the plug. Racing, aviation, and mass-production commercial plugs all use the same "Sintox" material. This material is formed into cylinders, which are afterwards machined to the required shape. These are then fired at a high temperature which fuses the material to a jewel-like hardness second only to that of the diamond. At this point a number of inspections are made to ensure complete freedom from defects, including examination under a fluorescent light which shows up the smallest imperfections with dramatic clarity. The outer surface of the insulator next has to be glazed and the electrode assembled in the hole already provided in the insulator.

The centre electrode is provided with a shoulder corresponding to a shoulder in the insulator hole, between which is inserted a patented gas-tight seal. An inserted wire joins the electrode with the centre pin running through the upper portion of the insulator and a space between the two is first filled with a sealing material. The assembly is then heated to melt this material, while at the same time pressure is applied to the centre pin, which compresses the molten material, forcing it around the centre pin and the electrode, thereby retaining them securely in position, to ensure gas-tightness. The next step is to secure the assembled centre within the steel body; here again, heat and pressure play their part. There is a gas-tight joint formed between the insulator and the steel body by a ground seating held together under pressure by swaging the upper edges of the body on to a shoulder formed on the insulator. Great care and precision attend all these operations and it is scarcely surprising that all Lodge racing plugs can pass a very severe test for gas leakage, during which a pressure of 600 lb. per sq. in. is applied to the plug, the temperature of which at the same time is raised to 200 deg. C.

The price of Lodge plugs is 5s. 6d. each, with the exception of the H-series, which cost 6s. 6d., the HLNR at 7s. 6d., and the platinum-pointed and racing pings, which cost 15s. each. A full range of terminals, insulated covers, etc., is listed.

Lodge Racing-plug Heat Range Chart

Lodge Heat Symbol                    Hot < (Soft)                    Cold > (Hard)

                                                47          49                     50-51          52-53

Approx. Continental Equivalent  300      310-380               400-440      450-500

14mm.     Normal    Non-           

               Reach     Detachable R47         R49                     R50            R53

               Long       Non-

               Reach     Detachable RL47       RL49               RL50 RL51  RL52 RL53

15mm.     Normal    Non-           

               Reach     Detachable                                         18-49           18-51                            

               Long       Non-

               Reach     Detachable                                        18L-49         18L-51

With Lodge racing plugs it is possible to start from cold, warm up the engine, and race, without the need for changing plugs. This can be achieved by following the approximate recommendations shown.

Normal Long          Reach Reach

14-mm. Thread:

R47                      RL47     Single point, non-detachable

R49                      RL49     Single point, non-detachable

R50                      RL50     Single point, non-detachable

R51                      RL51     Single point, non-detachable

                            RL52     Single point, non-detachable

R53                      RL53     Single point, non-detachable

18-mm. Thread:

18-49                   18L-49    Single point, non-detachable

18-51                   18L-51    Single point, non-detachable

It is recommended that Lodge racing plugs which require servicing should be returned for that purpose to Service Department, Lodge Plugs, Ltd., Rugby. Service charges are moderate.

Competition Manager: V . Martin-Jones.

Manufacturers: Lodge Plugs, Ltd., Rugby (Rugby 2076).


The present Pacy plug is the detachable version of the famous 3s. 6d. Wipac. It is priced at 5s., except for the 14H, a special sports plug for high-compression engines, which costs 6s. The 14T is recommended for less exciting o.h.v. and o.h.c. engines, and the commercial 18HT 18-mm. plug has given good results in vintage cars.

Makers: Wico-Pacy Sales Corporation, Ltd., Denbigh Road, Bletchley (Bletchley 320).


During the war the Pacy 5s. detachable sparking plug was absorbed in large numbers by the Services but with the return to peace a new design was decided upon to give the motorist a first-class product at a much reduced cost. Designed by D. A. Pacy, director of Wico-Pacy Sales Corporation Ltd., and late of Lodge Plugs Ltd., it underwent a prolonged period of testing. Both on the bench and in actual service on the road these plugs were tested, sometimes to destruction, for a period lasting well over a year. Only when their performance was satisfactory were they placed on the market and became, in a very short time, the well-known Wipac 3s. 6d. spark plug. Thus Wico-Pacy have the distinction of listing the least expensive sparking plug on the English market. They are available in 10-, 14 and 18-mm. sizes, have blue Aluminoex insulators and are guaranteed for 10,000 miles.

Makers: Wico-Pacy Sales Corporation Ltd., Denbigh Road, Bletchley (Bletchley 320).