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).
Technical report on piston performance, comparing Aerolite and standard pistons, and discussing issues like piston slap and oil consumption.
| Identifier | ExFiles\Box 132\5\ scan0058 | |
| Date | 11th March 1939 | |
| -6- PISTONS - Cont'd.{John DeLooze - Company Secretary} The main reason for going to the Aerolite piston, was the claim that they gave 8% more power than the R.R. piston. It is fairly certain, however, at the present time, that the standard Bentley II series 'H' Aerolite piston is no more efficient than the present standard Wraith piston. This, because it has been found necessary to decrease the skirt clearance from .003" down to the present limit of .0005" to "a fit". Recently L.A. has had experience of piston slap on the very latest production Bentleys due to tight pistons, and the only way to get over it was to fit pistons by selection in these cases. The reason for going to this tight skirt piston, was to overcome excessive oil consumption. At the time, endless experiments were carried out in an endeavour to control the oil by efficient piston rings, and we have to admit that this factor of proper oil control by the rings (not the piston) is the thing we know least about. So much for the skirt. The piston crown of the latest pistons, both R.R. and Aerolite, are identical as regards land dimensions, overall diameter and raised crown. It was found necessary to alter the land dimensions because we had isolated cases of customers siezing a piston immediately after decarbonisation. This was due to accumulated carbon on the top land becoming overheated. At the same time, it was found that production were not grinding these pistons strictly to drawing, a fact that had not assisted matters, because they had carried the taper grinding of the intermediate lands right across the top land. This in effect, had given an extra .002 clearance across the crown of the top land, allowing the excessive accumulation of carbon to take place. TESTING FOR BORE WEAR. Up to the present time we have been attempting to accumulate wear figures from engines doing all types of running both in and out of chassis. These we separate as follows. | ||