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).
Comparative table of exhaust and inlet valve and stem dimensions for various engine models.
| Identifier | ExFiles\Box 96\2\ scan0065 | |
| Date | 27th September 1935 guessed | |
| EXHAUST Dia Valve. | Dia. Stem. | % Present 20-25 HP. | 1.2 | .343 | 28.4 Present Bentley. | 1.3 | .343 | 26.4 25-29 HP. | 1.3 | .343 altered to .375 | 26.4, 28.8 4 litre Bentley. | 1.4 | .343 altered to .375 | 24.6, 26.7 SpectreCodename for Phantom III. | 1.25 | .343 | 27.5 SpectreCodename for Phantom III Proposed. | 1.25 | .375 | 30 INLET Dia. Valve. | Dia. Stem. | % Present 20-25 HP. | 1.35 | .312 | 23 Present Bentley. | 1.40 | .343 | 24.6 25-29 HP. | 1.5 | .343 altered to .375 | 23, 25 4 litre Bentley. | 1.6 | .343 altered to .375 | 21.5, 23.4 SpectreCodename for Phantom III. | 1.45 | .343 | 23.7 SpectreCodename for Phantom III Proposed. | 1.45 | .375 | 25.8 | ||