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).
Physical changes in 20/25 pistons due to heat treatment during engine operation, and the importance of a leisurely running-in schedule to prevent stress cracks.
| Identifier | ExFiles\Box 86\2\ scan0166 | |
| Date | 28th November 1933 | |
| - 2 - E.4/HP.28.11.33. ---------------- There is not the slightest doubt that if we took a set of these 20/25 pistons today newly fitted to an engine and opened up more rapidly than our standard running in schedule allows the result would not be satisfactory. in the case of the Bentley we are inclined to think that incremental loading is of still greater importance. After the piston has been in use we know that a Brinell test taken from the centre of the crown to the edge and down to the bottom of the skirt would show a decided change in the physical state of the material which follows the heat gradient occurring along the line described. It appears to us, therefore, that the piston undergoes an ageing or tempering treatment in the engine and in view of the varying sections which occur in the casting by virtue of its shape and design and the sudden local temperature gradients which may exist owing to this, it is of the utmost importance that these physical changes should be of a sufficiently leisurely nature to avoid any question of stresses being set up thereby and causing cracks which are otherwise very difficult to explain. E.{Mr Elliott - Chief Engineer} [Signature] | ||