Technical evaluation and test report on the 'PRIMA' sparking plug.
| Identifier | ExFiles\Box 33\3\ Scan197 | |
| Date | 26th November 1921 | |
| "PRIMA" SPARKING PLUG. These plugs were supplied to us for test by - Mr. Geo. Roberts. General Construction. The plugs are made with mica insulation - the centre insulated portion of the plug being detachable. The sparking plug gaps are not adjustable - the gap is formed by a disc having 6 sparking points. Criticisms. We do not consider that the mica insulation is held in a satisfactory manner. The insulation is a series of mica washers and wrapping which is clamped together by means of the long central electrode. This method of holding the mica insulation has been used by several plug makers and condemned. The reason being that for mica to form a satisfactory insulator in a plug when used in the form of washers, it must be compressed very tightly or else the oil percolates between the washers and forms carbon and breaks down the insulation. We should not approve of any plug in which the insulation was held in the manner it is on the Prima Plugs. The plugs were supplied - 6 with a gap of .5 m {Mr Moon / Mr Moore} /m - and 6 with a gap of .75 m {Mr Moon / Mr Moore} /m. As they were received we checked them up and found that the gaps in the plugs varied on those which were intended to be .5 m {Mr Moon / Mr Moore} /m from .022" to .018 and .025" - .030". We proved also that if the plugs were taken apart, extreme care had to be taken in replacing them or else the centre electrode of the gap became out-of-square and so altered the sparking gap considerably. We ran six of these plugs for 50 hours on an engine bench test. We tested them on there for pre-ignition, over-oiling and reliability. We found that the plugs were quite satisfactory as regards pre-ignition. When examined after they had been run in a hot condition, we found, however, that the sparking gaps had be altered owing to the centre electrode moving during the run. The plugs were also reasonably good for resisting oil. We have run the other six plugs for 2,500 miles on the road. The trouble we had with them on the car was that the sparking gaps altered and spoilt the slow running of the engine. contd:- | ||
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).