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 testing, comparing different heat treatments, dimensions, and grinding specifications to diagnose engine knocking.
| Identifier | ExFiles\Box 86\2\ scan0081 | |
| Date | 18th October 1933 guessed | |
| -2- XX We have taken from production a set of pistons to EB.1025 which are the present standard Bentley piston and grinding of which is similar to those used by GWH.{George W. Hancock - Head Chateauroux} in No.4. unit, the heat treatment however is standard. The Brinell on the crown of these pistons before running was 100 the hardness number at both top and bottom of the skirt was 106. The present standard Bentley piston if correct to drawing should be exactly bore size after running, at the base of the skirt. The standard 20/25 pistons E.57122 ground to E.55350 if correct to drawing should be .002 under the bore diameter, so that actually none of our Bentley pistons are exactly similar in dimensions to standard 20/25. The standard 20/25 piston is of course approx. .750 less in length than Bentley piston. We have, as stated previously, had this piston trouble when using specially heat treated pistons ground to Ex.11480 A.{Mr Adams} which grinding allows the dimension at the base of the skirt to be .005 under the bore diameter. We have not yet tried pistons to Ex.11480.A grinding and using the standard heat treatment, so that we cannot say definitely whether the new heat treatment or the fact that the pistons are so much under the bore size, is the cause of the knocking. We have however run a set of standard heat treatment pistons, to EB.495, with grinding and bedding as suggested by Mx.{John H Maddocks - Chief Proving Officer} (the dimensions of top of the pistons above skirt - i.e. lands etc, were as standard 20/25), this bedding is very similar to Ex.11480.A and this piston ran for 16 hrs. before any knocking was noticed. This is of course considerably longer running time than with pistons with special heat treatment, these two sets as mentioned earlier in this report only ran for 1 1/4 hrs. and 15 mins. respectively. This rather points to the fact that the new heat treatment is in some way, not yet clear to us, inferior to the standard heat treatment. The grinding of the piston is also partly responsible for the trouble as shown by the fact that pistons ground to the Mx.{John H Maddocks - Chief Proving Officer} type of bedding began to knock after 16 hrs; we have run pistons in 4.B.IV. chassis (No.3. unit) ground to Ex.11480, which grinding is very | ||