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).
Technical memorandum discussing the issue of piston knocking, comparing cast iron and aluminium pistons, and outlining causes and potential cures.
| Identifier | ExFiles\Box 35\2\ scan 121 | |
| Date | 1st September 1919 | |
| Contd. -2- We never pressed for the acceptance of pistons which knock when the engine is at a satisfactory working temperature, and I do not see the slightest reason from this point of view to revert to cast iron pistons which have never been free from defect, and if we do revert to cast iron pistons I cannot possibly agree to the past practice of fitting these with unsafe clearance. They must have at least a minimum of .004" clearance instead of a minimum of .002" and .003" which has been the practice of the Test Dept. to fit to meet Sales' requirements. Moreover, cast iron pistons are reported to be liable to still knock with a warm engine, whereas the aluminium ones are thought by us to be better in this respect. Referring to 1.EX the pistons on this engine are quite the noisiest aluminium pistons I have ever had on a car, but even with these the knock becomes imperceptible after quite a few minutes of warming up, after which, in my experience, there is not the slightest ground for complaint. It will be remembered that this warming up running time is quite a necessity to get the engine in a state when it will carburate the incoming air satisfactorily. Mr. Hives has discovered that the cause of variation of one piston knocking and another not knocking with the same amount of clearance, is brought about by the following condition The pistons which knock are those which are most truly lined, and perfectly fitted so that there is no twist of the piston by the connecting rod, and he informs me that he can cure any piston knocking by getting it slightly out of alignment by the aid (Contd) | ||