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).
Testing letter box slot and Zephyr type pistons, detailing issues with clearances, seizing, and knocking.
| Identifier | ExFiles\Box 77\3\ scan0053 | |
| Date | 16th December 1919 | |
| To R.{Sir Henry Royce} from Hs.{Lord Ernest Hives - Chair} c. to CJ. c. to BN.{W.O. Bentley / Mr Barrington} c. tp E.{Mr Elliott - Chief Engineer} c. to DA.{Bernard Day - Chassis Design} X3432 Hs{Lord Ernest Hives - Chair}5/LG16.12.19. X.3732. RE PISTONS. X.3751. We have tested present standard pistons with a letter box slot. We find that with the letter box slot we can-not run with anything like as fine clearances as we are running with on the standard split pistons because of seizures. The safest sizes we could get down to, when we had a piston with a letter box slot one each side, was .007" at the top of .004" at the bottom on the fitting portion. With these clearances we got definite pistons knocks when the engine was cold and occasional knocks when the pistons were hot. We think if the piston was designed for the letter box slot they would no doubt work much better. We have also been carrying out tests on a special type of Zephyr piston (got out by BN{W.O. Bentley / Mr Barrington}) on which the skirt is made so that it can easily collapse when it expands. We have run some of these pistons and fine that we can run with very fine clearances without seizing. With these fine clearances, we still get piston knocks both when the engine is cold and hot. We feel that the trouble with these pistons is that they are too flexible. Tests we have made with them, by loading them up, confirms this. We also have had trouble in grinding these pistons because when they were taken off the machine, the sizes altered. We have by no means exhausted the possibilities of these pistons. we are trying some more with still tighter clearances and if necessary , we can go | ||