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).
Analysis of experimental tests on mica spark plugs, detailing their faults and comparing them to ceramic alternatives.
| Identifier | ExFiles\Box 162\3\ img042 | |
| Date | 4th December 1936 | |
| -2- Our opinion as a result of experimental tests with these plugs is that the rough edges of the mica become incandescent at a certain temperature dependent on engine speed, and early ignition results. This trouble in a general way only occurs in wrapped mica plugs i.e. where the mica is rolled round the centre electrode as in the K.L.G. 765 plug, but we have had one case where the same trouble occurred with a mica washer plug e.g. the K.L.G. 850. In this case the mica washers immediately underneath the electrode head had become displaced and the exposed mica edges caused the same effect. There are therefore six fundamental faults with mica insulated plugs as follows:- 1. Flaking of the mica on wrapped plugs. 2. Pin holing and washer displacement on the washer construction plugs. 3. More prone to formation of lead deposit. 4. In order to clean the plugs a two piece construction is necessary. 5. They are more prone to condensation. 6. They are expensive to manufacture. Dealing now with Ceramic plugs, such as the Champion, A.C, Lodge and Bosch, we have samples of each of these plugs now undergoing test. We have already eliminated the Lodge as we experienced insulator fractured and we know that the mechanical properties of 'Steatite' as used by Messrs Lodge are not at all satisfactory. The A.C. and Bosch plugs both employ "Sinterkorund" insulators which are three times as strong mechanically as the Champion. Messrs K.L.G. have acquired the Bosch rights and already have a plant in operation at Putney. By the end of the year they have promised us the first production samples. They have spent £25,000 in setting up this new plant. The A.C. Company have not yet acquired a production plant and all the plugs we have had from them have been made in a small continued | ||