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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).
Letter to Mr. Claude Johnson regarding piston design modifications to prevent engine knocking and cramping.

Identifier  ExFiles\Box 148\3\  scan0007
Date  24th March 1921
  
Oy10 - C 24321

Mar. 24, 1921

Mr. Claude Johnson - Sheet 3.

For example, we find that after a new engine has been run an hour or so (to run in the timing gears) the pistons will sometimes show .006 slack in the cylinder. We therefore consider that the clearance holes in the piston bosses should be increased from .265 diameter to .312 diameter so as to avoid any possibility of cramping the pistons.

As an example of the troubles which arise with the present type of piston, we may mention that on 22-LW (our first English demonstration car) we have had to peen out the pistons four times in 9000 miles of running, each time to prevent knocking which had developed. This is not elusive knocking but sheer piston slap.

We should like the opinion of Mr. Royce and the Works Committee on this suggestion, and their comments on the piston design of the Aluminum Manufacturers, Inc.

MO/C

Yours truly,
ROLLS-ROYCE OF AMERICA, Inc.
Per M.{Mr Moon / Mr Moore} Olley

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