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).
With a sketch discussing design modifications and compression ratios for a 40/50 HP chassis aluminium cylinder head.
| Identifier | ExFiles\Box 65\4\ scan0384 | |
| Date | 22th March 1928 | |
| Hs.{Lord Ernest Hives - Chair} C. BY/CS. X8050 BY4/K.{Mr Kilner} 22. 3. 28. 40/50 HP CHASSIS ALUMINIUM CYLINDER HEAD. In the early development of the aluminium cylinder head you requested low compression pattern to be prepared so that it would produce both the 4¼ to 1 and 4½ to 1 compression ratio cylinder heads. The 4¼ to 1 ratio has been instructed for production which means the floor of the casting is .150 thicker than the original design. Do you still contemplate any increase in the compression ratio or can we instruct for our core boxes to be permanently altered to suit the 4¼ ratio only. The sketch below shows the pattern affected. It should also be noted that another pattern produces both the 4¾ to 1 and the 5 to 1 high compression cylinder heads. Text from sketch: 4½ to 1. 4¼ to 1 .150 .225 .375 BY | ||