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).
Letter discussing Bentley valve stretching, wide plug gaps, and tappet clearance experiments.
| Identifier | ExFiles\Box 126\4\ scan0116 | |
| Date | 28th April 1936 | |
| G.W. Hancock, Esq., Hotel de France, Chateauroux, Indre, France. X1104 Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}18/KW.28.4.36. We have sent your axle by Air Mail. We note your remarks re valves stretching on the Bentley, and think it would be as well if you brought the Bentley water circulation up to the 20/25 standard by putting all the water in at the back of the head. We should like you to be quite positive that the valves are stretching before you throw them out because we should imagine it is very difficult accurately to measure .003", which is the maximum amount of stretch you record. We have had Taub, the Chief Engineer of Chevrolet's, at the factory for a couple of days, and he says that it is possible to get excellent slow running and improved economy by running with really wide plug gaps. He astonished me by saying that they were running with .030" on the Chevrolet standard, and were making a big effort to get up to .040", the trouble being to get the coil people to come into line. He said that a big plug gap would give decent slow running but thoroughly bad distribution. His claim certainly sounded somewhat fantastic but we should like you to see whether you can run with .030" plug clearance with our existing electrical equipment, and if so whether you do gain anything in consumption and idling. Taub says that they dare not let their exhaust valve clearance get to less than .003" under flat-out conditions. We know that on the test bed with continuous blind the exhaust valves are barely on their seats if we run with .008" tappet clearance. We suggest that you run 3 of the 20/25 exhaust valves at .008" and 3 at, say, .005" and see if there is any material difference in the condition of the valves after the run. You might also let us know whether with .008" clearance the engine is very noisy. The Phantom III is going well and we shall have it in France before long. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||