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).
Investigation into cracks found in a Wraith model cylinder head.
| Identifier | ExFiles\Box 8\6\ 06-page37 | |
| Date | 14th September 1938 | |
| Tg. from Rm.{William Robotham - Chief Engineer} c. Hs.{Lord Ernest Hives - Chair} c. Ey. c. RHO. c. Mx.{John H Maddocks - Chief Proving Officer} c. Ba. 619 Rml/R.14.9.38. CRACKS IN WRAITH CYLINDER HEAD. Further to Ey/W.10.9.38., we have been investigating the cracks reported on 27.G.VI cylinder head after 6,300 miles. These are the first cracks that have ever been detected on a Wraith cylinder head which has been run only under road conditions. At first sight, therefore, one concludes that the fitting of small exhaust valves has caused the trouble. However, on examining a large valve Wraith head from an engine on which we have been carrying out cylinder bore wear test, we find that cracks have appeared in exactly the same position on the exhaust valve seat, and, furthermore, an inlet valve seat is cracked. This engine has been subjected to abnormal treatment in that it has been making repeated starts from cold, and immediately on starting has been run under full throttle high speed conditions on each occasion. It is, therefore, clear that the large valves are not a solution to the trouble. The Wraith cylinder head differs from the heads we have been running for the last five years on -J's and Bentleys in that the disposition of the material round the combustion chamber has been modified. The modification was adopted as a result of tests which showed that it reduced the tendency to detonate. A head of the old type has been run on the Test Bed in parallel with the Wraith head which has failed and has done the same sort of work. In this case the valve seats have not cracked. The story, therefore which we have so far obtained is obviously as yet incomplete, but in order to clarify the production position we tabulate the conclusions so far drawn from the evidence available :- (1) Cracks in valve seats have come up since we modified the casting on Wraith Cylinder Heads to reduce detonation. They are however, limited to three heads out of some fifteen in use experimentally. (2) These cracks were not detected initially because they do not show up under ordinary running conditions, i.e., the head which successfully completed 17,000 miles on 26.G.VI. in France was entirely free from cracks. | ||