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).
Issues with Bentley cylinder bores, comparing honing to grinding and the performance of different piston rings.
| Identifier | ExFiles\Box 130\2\ scan0334 | |
| Date | 7th December 1936 | |
| To Hs.{Lord Ernest Hives - Chair} from Rm{William Robotham - Chief Engineer}/GWH.{George W. Hancock - Head Chateauroux} RHC.{R. H. Coverley - Production Engineer} Rm{William Robotham - Chief Engineer}/GWH.{George W. Hancock - Head Chateauroux}11/AP.7.12.36. BENTLEY CYLINDERS. Working in conjunction with RHC.{R. H. Coverley - Production Engineer} and Rm.{William Robotham - Chief Engineer} two cylinder blocks were finished with the exception of honing. It was arranged to have them ground by Harris of Kilburn, who now grinds all rebores. Upon fitting the blocks on their rig and very lightly touching the bores it was found that the bores were not true to the bottom face. The centre of one bore was above standard size. The honing of these bores could not possibly have corrected the inaccuracies. To bring the bores true it was necessary to grind them .005" oversize. As we have no pistons this size they had to be ground .010" oversize. The bores when finished were true to 1/100 of a millometer measured at Kingsbury. The result of this is as follows - One block has been fitted to Mr.Ferguson's Bentley B 86.GA. The oil consumption on his car prior to fitting the new block was 600 M.P.G. With the new block with 200 miles running only 1600 M.P.G. has been obtained. The engine is smoother at high speeds. We made one exception to standard practice as follows - Before fitting the block the fit and clearance of the piston pressure rings was carefully observed. The R.R. rings supplied with the pistons were not at all a good fit. They were not round and a blue test shewed poor bedding. A set of standard Brico rings were tried and the fit was almost 100%. The difference in the fit requires seeing to believing. The pressure of the Brico rings are considerably less than R.R. We fitted all Brico pressure rings and R.R. scraper. The Brico rings commercial limits are 9 to 12 lbs.sq.in. pressure. Those fitted were 11 lbs. sq.in. We were therefore with lighter rings more liable to over oiling and poor consumption, but the results have proved very much otherwise. I am confident that if the cylinder bores were ground instead of honed we should overcome our terrible complaints of overoiling and poor consumptions. Rm{William Robotham - Chief Engineer}/G.W.Hancock. N.B. The measuring of bores does not shew up the faults. | ||