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).
Letter to The Light Production Co. Ltd. about returning a set of cracked pistons that failed during a high-compression engine test.
| Identifier | ExFiles\Box 87\2\ scan0170 | |
| Date | 7th April 1936 | |
| Exptl. Dept. Hs{Lord Ernest Hives - Chair}/Mths.{Reg Matthews}15/KW. 7th April, 1936. The Light Production Co.,Ltd., 60/66, Rochester Row, Westminster, LONDON, S.W.1. For the attention of Mr. C.H. Brooks. Dear Sirs, We are returning to you under separate cover a Piston that has been running in a high compression engine and in which cracks have developed after 13½ hours running at full throttle. The Piston is of the type standardised for production and is cracked in the usual place; i.e., through the ring lands and up to the crown. All pistons in the set are cracked and all show signs of tightness; we are surprised that the pistons show signs of tightness since they have been run in bores .002" oversize, which has been our usual practice with our high power units. We are replacing the cracked pistons with a set having the heat break sawcut in the scraper ring groove, and at the same time honing out the bore a further .001", this has been necessary in order to clean up the bores and our clearances then should be .008" at top of the skirt and .004" at bottom. Do you consider the clearance at the top of the skirt excessive and likely to lead to cracks developing in the skirt due to side slap ? This type of piston we know to have about 50% longer life than the previous set, but we shall require a piston life of 30-35 hours in order to be safe. The piston we have standardised for the 4¼ litre Bentley we believe has been made to approximately the same sectional thickness as the lightweight piston we used on the 3½ litre Bentley and which was subsequently turned down as being of too light a structure. Since then, however, the large bore piston has been strengthened in one or two vital spots, and is satisfactory for normal purposes. | ||