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).
Modifications to the Wraith piston due to issues with carbon build-up and grinding.
| Identifier | ExFiles\Box 148\1\ scan0009 | |
| Date | 22th November 1938 | |
| 1256. To By.{R.W. Bailey - Chief Engineer} c. to Dn. c. to RHC.{R. H. Coverley - Production Engineer} Rm{William Robotham - Chief Engineer}/CTS.{C. Trot Salt - Carburation}6/JH.22.11.38. PISTONS. Further to our report Rm.{William Robotham - Chief Engineer}5/R.4.10.38. As a result of tests carried out in the above report, the Wraith piston has been modified by narrowing the top land, and increasing its diameter. Briefly after examination of pistons from a customers car, which had seized after being decarbonised. It was evident that too much carbon was collecting on the top land, which was not being subsequently removed during decarbonising. The extent of this layer of carbon was therefore the cue for a little extra metal on the diameter of the top land. With reference to the grinding drawing of the 25/30 piston, each land is a different diameter. The top land however, is supposed to be parallel over the extent of its width. By some verbal agreement, the Works have carried the taper grinding across all the lands. Approximately .002 has been lost from the final diameter of the top land therefore. This has been going on right through the whole series of 25/30 chassis, in fact since the bore was increased from 3.250". The drawing has never been altered, so that it is by a mere chance that the fact is revealed. It does not seem a feasible proposition to grind according to the drawing however. Because to produce the parallel portion on the crown, the piston would have to be removed from the machine, and this is undesirable for obvious reasons. We therefore suggest that the Wraith piston should be reconsidered and that the Aerolite method of having the lands all the same size be adopted. Rm{William Robotham - Chief Engineer}/CTS.{C. Trot Salt - Carburation} | ||