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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).
Suspected pre-ignition issues with 'Mercury' plugs due to their porcelain design.

Identifier  ExFiles\Box 33\3\  Scan116
Date  9th March 1921
  
X1812 C.
To HS.{Lord Ernest Hives - Chair} from R.{Sir Henry Royce}
Copy to CJ.

D
R4/CE/8/21.
RECEIVED
9 MAR 1921
HS{Lord Ernest Hives - Chair}

X4295 RE "MERCURY" PLUGS.

I have a suspicion that one of these on the small pump engine here was causing pre-ignitions after being in use many hours. Kindly beware of this, and if you confirm this, it is probably due to the wall of porcelain being too thin, and too large in diameter, or too high, and so giving a very large heating surface for its cooling conductivity.
So far the Champion has not shewn the same fault. It is possible, however, that the Mercury has partly cracked its porcelain in such a way that the walls are more isolated than when new.
If my suspicions are correct, it is highly advisable that we ask the makers to alter the proportions of the porcelain for those that they are supplying to us.
This matter could easily be tested by running the plugs hard in one of our aero engines, but this probably is too severe a test. One could therefore bring the severity of the test about right by comparing the Mercury plug with some other porcelain plugs which are satisfactory, including in the list the Champion, which may have a very lucky proportion in the design of the porcelain.

R.{Sir Henry Royce}
  
  


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