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 design choices, including ring types, overoiling, skirt flexibility, and grinding specifications.
| Identifier | WestWitteringFiles\T\2July1928-December1928\ Scan142 | |
| Date | 16th October 1928 | |
| HS.{Lord Ernest Hives - Chair} FROM R.{Sir Henry Royce} C.c. to BJ. WCR. (struck through) re. PISTONS. ORIGINAL R5/M16.10.28. x7040 x5040 (struck through) x8040 x741 (struck through) We conclude that you do not want a lower scraper ring. We agree we only put it on to reduce the overoiling tendency, which led to the adoption of 'L' shaped rings. We now understand that both cars will run with plain rings, and a perfect circle, providing the latter is ground out wide in the centre, having the effect of increasing the unit pressure. Regarding the number of rings I think it is hardly reasonable to conclude that a total of 4 rings is insufficient for our very moderate compression ratios. It will be realised that a perfect circle ring is worth half a ring. As we are anxious to keep the weight of the piston down, and to give as much bearing area as possible for the skirt, we would like you to persevere with the 4 ring type that we have lately sent you. The rings mentioned above are capable of being lapped, and so if carefully fitted, and all of them pinned, ensure a gas tightness greater than unlapped rings, with only part of the number pinned. Regarding the flexibility of the skirt we feel that with reasonable lubrication and the best system of grinding, this should give no trouble in the recent pedestal type. Our impressions are that it is difficult to keep the bottom of the skirt stiff enough, and we disagree with the idea of cutting away the lower stiffening ring. is The idea that the grinding should be approximately parallel, and slightly eccentric with the head so that the flaps of the split stand proud. Owing to the want of flexibility of the side it is understood that these have to have extra clearance by grinding taper from the top downwards - (LeC. 2554.) We want 3 narrow rings and a perfect circle ring all pinned as shewn on LeC. 2490. for both chassis. We understand you are following LeC. 2577. type B. for the 20HP. The following are the features of the pistons we wish to get adopted:- (1) | ||