The actual type of Plug as fitted to the Velocette was selected only after the most searching and prolonged tests and any sparking plug replacements that may ultimately be necessary should be of this same make and type.
To clean the sparking plug it must first be removed from the engine and the insulated centre electrode (A) unscrewed from the body (D) by means of the gland nut (B). This is best and most easily accomplished with the K.L.G. Combined Detacher and Plug Box Spanner, available from our Service Dept., but alternatively place the plug upside down in a vice, grip the hexagon (B) and use a box spanner on the hexagon of the body (D).
When (B) has been unscrewed the centre electrode may be extracted from the body.
Irrespective of type the interior insulation (C) should be wiped with a rag soaked in petrol, care being taken not to damage the mica. Whilst the actual firing point of the centre electrode may be scraped and, if necessary, polished with emery paper, no abrasive must ever be used on the mica insulation in any circumstances. If the deposit on the mica insulation should prove difficult to remove, the centre electrode should be left steeped in petrol, until a further application of the rag will disperse the deposit.
Under no circumstances must a Sparking Plug with internal mica insulation be cleaned, on any fixture or apparatus which employs sand blasting as a cleaning medium. Such treatment will rapidly ruin the insulation.
When the centre electrode has been thoroughly cleaned the body should be well scraped out and the mouth of the plug and the earth points brushed with a wire brush, the body being finally washed out in petrol.
Before re-assembling, verify that the interior washer in the plug body is in place. Care should be exercised when inserting the centre electrode into the plug body and the gland nut must not be overtightened.
The earth point, or points, should now be set to the correct gap of fifteen to eighteen thousandths of an inch (.015" to .018") by bending the earth point, or points, towards the firing point of the centre electrode or levering outwards as the case may be. The firing point of the centre electrode must not be levered towards one or more of the earth points.
The type of Sparking Plug fitted in the engine is intended for use under normal touring conditions. If it is the intention of the owner to use the machine for trials work or any kind of racing, or to " hot-up " the engine in any way whatever, such as by removing the compression plates then it is highly probable that a Sparking Plug possessing a higher internal heat resistance will be necessary.
It is impracticable to publish a recommendation list of racing types of plug, because every engine's sparking plug requirements need individual consideration. In such cases application should be made to the Technical Department of the Plug Makers, accompanied by fullest details concerning compression ratio, fuel, type of event, etc., etc. (for addresses see another page).