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).
Piston scoring caused by carbon deposits and recommendations for increasing clearance.
| Identifier | ExFiles\Box 77\3\ scan0020 | |
| Date | 11th November 1919 | |
| Contd. -4- EHL/LG7.11.19. This line corresponds with the amount of pistons run out. This, as a rule, is not alround the pistons but only in local places. The only medium we know which could cause this scoring or form this definite band, is the carbon deposit in the cylinder. We imagine that small pieces of carbon are pushed off by the piston and become jammed in the clearance between the piston and the cylinder wall. Low compression pistons, which do not project into the cylinder, we have not been able to find this distinct line but there are mild, harmless markings at local places. The first distinct case we had of this was on 7-CA. Mr. Royce will remember that this car knocked badly when run full throttle at high speeds. The pistons on there ran into the counter bore .125" and shewed signs of being very tight on the top band. (these pistons were the Zephyr type). We found another excellent example on one of the Shop cars. On there the pistons have run 11,800 miles and they shew the same distinct mark. We are sending to Mr. Royce a piston off 7-CA which illustrates what we mean. When we first saw the piston off 7-CA we increased the clearance on the top band of the standard pistons from .020" to .025". This was undoubtedly a step in the right direction and we anticipate that for ordinary car use, this may be sufficient, although in view of the information we have obtained from the bench test, we now recommend that the clearance should be still further increased. We have examined a number of the present cars - 1.EX which has run 4000 miles, although it has distinct marks on the top band, they do not look as though they would ever cause a Contd. R.R.C. 235 A (100) (T) S P.L. 376. 16-4-19) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 2384. | ||