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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).
Mileage at which bottom decarbonising and reboring of Bentley engines becomes necessary.

Identifier  ExFiles\Box 107\2\  scan0144
Date  17th June 1937
  
COPY.

K/Sr.
VA.

PK/KC/DPB/17.6.37.

Sr/VA4/ET.15.6.37.

re: R.20 BOTTOM DECARBONISING OF BENTLEY ENGINES.

Up to a few months ago we were carrying out an R.20 at 20 to 25,000 miles, changing all rings as a routine matter, but having come up against the same trouble that you mention, we try and get away with another R.10 at 20,000 miles and the R.20 at 30000 miles, unless there is an owner's complaint of heavy oil consumption or our incoming test shews that it is necessary for the block to be removed, simply due to the fact that at 20,000 miles the cylinder wear factor in some cases is approaching that figure which precludes the possibility of getting away with a change of rings only with any degree of certainty and without the bogey of heavy oil consumption looming up.

We find that from the figures taken by us over the past two years or so, many 3½-litres become borderline cases for reboring 'round about the 30,000 mile mark, and I doubt very much whether you will get many 3½-litres that will reach the 40,000 mark without having reached the wear factor figure (.008" .002" taper and ovality) at which a change of ring would be sheer waste of time and a rebore has become a necessity.

PK/KC.
  
  


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