The type of sparking plug ﬁtted as initial equipment has been selected after exhaustive tests and has the characteristics most suited for general purposes in this type of engine. Sparking plugs are obtainable from several manufacturers who each produce a range of plugs of varying diameters, threads, and reaches (lengths of thread) and each size is usually offered in a range of different heat resistances. At the one extreme, plugs are capable of withstanding very high working temperatures without overheating, and at the other extreme of being able to continue ﬁring in very oily conditions.
The ﬁrst will usually be found to oil up readily, and the second will not work long in a high performance engine before overheating sets in and causes pre-ignition of the charge in the cylinder. Thus, if a plug of the same make and type as that originally ﬁtted by us is unobtainable, the replacement must be the same diameter and reach and must have similar characteristics. This is very important as, whilst a plug of too high internal heat resistance will usually only cause difficult starting, and misﬁring, serious harm can be caused to the engine by the use of a “soft” plug, that is one which becomes overheated easily.
Our recommendations for the MSS are KLG type FE80, Lodge type HLN, and Champion type NA8 and for Venom and Viper models KLG FE100, Champion NA10 or Lodge SHLN.
The thread diameter is 14 mm. and the reach 18 mm. (extra long). In no circumstances whatever must plugs of different reach be ﬁtted.
Sparking Plugs for Extra Hard Duty and Racing.
The types of sparking plugs recommended above are those suitable for normal road running.
Many Viper and Venom models are driven exceptionally hard and are also being used to an increasing extent for racing. For such purposes plugs having higher heat resistance are often necessary.
For really hard sustained high speed driving on the road with silencer ﬁtted the K.L.G. type FE220 is desirable.
For racing on short circuits where, although speeds are high, the engine is frequently throttled back, the K.L.G. type FEZSO is often suitable.
On longer circuits for Clubman racing where there is the opportunity to use full throttle for long periods the K.L.G. type FE280 will be required.
These recommendations must necessarily be in the nature of a guide only. The correct type for any given set of conditions, apart from normal use, must be decided ﬁnally on the course after taking plug readings during practice runs before the race.
The plug will require cleaning from time to time and the method used will depend upon whether the plug is constructed so that it may be taken apart for cleaning or is of the “non-detachable" type.
The " non-detachable " type must be cleaned by brushing the electrodes and the insides of the gas space as far as possible with a wire brush, or by "sand-blasting" on one of the special sparking plug cleaning machines. In either case the plug must afterwards be scrupulously cleaned out by washing in petrol to eliminate all risk of dirt, and more particularly, the sand used in cleaning, from working into the combustion chamber during subsequent service. Do not in any circumstances reﬁt a sparking plug immediately after cleaning on a Plug Service Cleaner employing a sandblast, but wash it well in petrol, and blow out with a compressed air jet.
To clean a “ detachable " type plug, take it apart by holding the gland nut ﬁrmly, but not so tightly as to cause damage by crushing, in a vice—the jaws of which have been covered to prevent marking the nut. The plug body is then unscrewed off the gland nut. On removing the central portion take care of the sealing washer which must be reﬁtted on reassembly
Scrape the electrode, insulator, and the inside of the body clean of carbon with a penkriife or clean with a wire brush. Rinse in petrol before reassembling, make sure that the copper washer is in good conditions and smear lightly with thin oil. Reﬁt carefully and put the centre into the plug body, tightening the gland nut fully.
Adjusting the Plug Points.
After cleaning. the ﬁring points will require adjusting to a gap of .018-in to .023-in. Never adjust by bending the centre electrode as this will crack the insulator and render the sparking plug useless. All adjustments must be made by setting the side electrode.
Reﬁtting to the Engine.
Make certain that the threads are quite clean and, if the sparking plug has been cleaned on a “ Sand-blast ” cleaner, that there is no sand left inside the plug—this is most important.
Lightly smear the threads of the plug with graphite paste or “ Oil Dag," and screw in to the head, being most careful to avoid cross threading.
Do not overtighten.