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).
Report investigating trouble with piston knocks on Bentley engine units.
| Identifier | ExFiles\Box 86\2\ scan0080 | |
| Date | 18th October 1933 | |
| FILES: X-36 18.10.33. BENTLEY PISTON KNOCKS. We have recently been experiencing trouble with piston knocks on Bentley units. The trouble has been confined to No's 6 & 7 units on the test beds, and in each case the pistons fitted were EB.495, specially heat treated, and ground to Ex.11480A. The piston knocks were first noticed after running the first Power Curve on No.6 unit, which corresponds to 15 mins. running with the engine developing 90 B.H.P. or more. No.7 unit had run one hour's endurance besides its power curve, which makes the total full throttle running 1 1/4 hrs, before knocking was noticed. We cannot say whether the piston skirts have collapsed much on No.6 unit, because we do not know the actual dimensions of the pistons when fitted to the engine, we have the dimensions since running and these conform fairly well to the drawing dimensions. The Brinell hardness number of the crown of these pistons averaged out at 123, whilst the figures for the top and bottom of the skirt were 106 and 95 respectively. GWH.{George W. Hancock - Head Chateauroux} has reported piston knocks on No.4. unit now in France on 10,000 miles test, here again the pistons have been specially heat treated, but in this case the pistons have not been ground to the same drawing. The pistons in No.4. unit are ground to Ex.11480 and are similar in every respect to pistons which have done 15 hrs. full throttle running in C.XI unit, except that the heat treatment is the special one as recommended by H1. The Brinell hardness number called for on drawing using normal heat treatment is 106 - 140. The pistons removed from C.XI (having normal heat treatment) after running 15 hrs. showed Brinell hardness of 73 on the crown, the hardness number at the top and bottom of skirt were 108 and 101 respectively. | ||