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).
The causes of engine knock related to piston fit, oil film, and piston design.
| Identifier | ExFiles\Box 66\1\ scan0064 | |
| Date | 12th June 1925 | |
| R.R. 493A (50 H) (D.D. 31, 12-6-25) J.H.D. EXPERIMENTAL REPORT. -2- Expl. No. REF: Hs{Lord Ernest Hives - Chair}/ACL/LGS.10.25. The knock, though to a much lesser degree, can be produced very simply by moving a piston attached to a con: rod up and down by hand in a cyl: bore which is heated to about 90°C. If the piston is a good fit having no fore and aft movement when cold it will be quite tight in the bore when hot, and will give a small but distinct knock when the direction of motion is changed. At first sight this seems rather impossible consider- ing the piston is a tight fit but the following explanation seems to fit the case. The piston although being a tight spring fit when hot preserves a film of oil between it and the bore when in motion but when the direction is changed the film at any particular tight spot may momentarily break down but is immed- iately restored when motion in the other direction starts. This breaking down of the film, it seems probable could give rise to the knock and actually the xxxx increased resistance when re- versing the directions is very noticeable. We have tried some pistons with more clearance and ground in the same way as is the practice with our American cars. The pistons were ground concentric and parallel and were relieved at the gudgeon pin sides. The bottom of the skirt was left concentric as it was thought that this may reduce the tendency to over-oil, but at present we cannot say if this is so. These pistons were an improvement with regard to knocks but it cannot be said that they entirely contd :- | ||