From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Motor' magazine describing the manufacturing process and testing of mica spark plugs, specifically the detachable 'G' type K.L.G. plug.
Identifier | ExFiles\Box 33\3\ Scan092 | |
Date | 29th December 1920 | |
The Motor 1115 December 29, 1920. THE MARVELS OF MICA.—Contd. indicated by a small pipe which runs from the press into a little bath of methylated spirits, the leak, of course, being indicated by a bubble. After this, thin circular washers are threaded over the upper portion of the mica roll and held in close contact by means of a nut. The electrode is then spun in a lathe and the uneven exterior of the washers sand-papered down until they assume the brilliant brown polish so well known. How Plugs are Tested. The electrode with its insulation is then tested. Unless it will withstand an electric current strong enough to produce a spark 1/4 in. or over 3/8 in. long in air it is rejected. A girl fits it into the holder and attaches a high-tension lead to its end, and the current is switched on. A jumping torrent of purple sparks leaps from the end of the electrode to the collar: and should the insulation be faulty the sparks do not appear, their places being taken by a dull glow in the mica, which indicates internal leakage. The Advantages of Detachability. Every single plug has to pass through this test, and it says much for the thoroughness of the manufacturing methods that less than 2 per cent. is found to be faulty in any stage. After this the electrode is assembled into the body of the plug. The new “G” K.L.G. is of the detachable type, and possesses several distinct advantages. The majority of motorists of long standing usually have a heterogeneous collection of plug bodies lying about their garage or workshop. It is only the insulation which is faulty, but unless spare central electrodes are available, the body of the plug, which costs quite a sum to manufacture, is useless. With the new K.L.G. spare central electrodes can be obtained at exactly half the price of a new plug, so that should one chance to become damaged the old body can still be used, with considerable saving in cost. Although the manufacture of the metal part or exterior of the plug is purely a mechanical process, it is certainly far from being uninteresting. Absolute accuracy is required in the cutting of the thread, so that no leakage and no damage to the cylinder of the engine can possibly occur. To check the thread formation, an instrument is employed which is in reality an optically correct magic lantern. The plug is placed in the rays of light and a shadow of the thread enlarged exactly 50 times is thrown on to a distant screen. Here the formation of the thread can be clearly observed. Any dust, almost invisible to the naked eye, on the plug itself appears like lumps of dirt 1/4 in. across on the screen. Motorcar engines come in for all kinds of misuse, and it has often been found that plugs rust into their cylinders. This was particularly the case on aero engines when steel cylinders were employed. To overcome this drawback the new “G” type K.L.G. plug has the body “sheradized.” By this means the exterior of the plug is given a coating of zinc by a special method, which allows it to penetrate some 2-1,000ths of an inch into the metal. This coating absolutely prevents rusting action being set up by water or damp, and also prevents the plug threads rusting firmly into those of the cylinders—a detail that will most certainly be greatly appreciated. Image Caption: Enlarged fifty times its proper size, a shadow of the thread is thrown on to a screen to enable it to be checked for accuracy. Compare the size with the actual picture of a sparking plug held in the hand. It will almost go between the shadows of the points. Ex-service men will be kept busy for five months in widening the road between Birmingham and the Austin Works at Longbridge. The photograph shows the bank at Pigeonhouse Hill, just being demolished in order to straighten out the narrowest part of the road. c4 | ||