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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).
Various thread standards used for name-plate screws, studs, and bolts in automobile construction.

Identifier  ExFiles\Box 33\1\  Scan255
Date  27th October 1925
  
TO C.I. FROM R.{Sir Henry Royce}

Copy to - BL
" GY
" HS.{Lord Ernest Hives - Chair}

ORIGINAL X1775

RS{Sir Henry Royce's Secretary}/M27.10.25.

re. NAME-PLATE SCREWS. X1775

These are not a special RR. pitch, as we have no special sizes of studs and bolts, because we use the British fine thread standard for 1/4" and over inclusive, and the British Association standard for all those under 1/4" exclusive. These standards we have always used since the beginning of our automobile construction, and we believe that these fine threads are necessary to enable us to compete with continental and American productions, who also use a finer thread than the old fashioned Whitworth standards.
It will be understood then, above 1/4" these are Whitworth pattern threads, but of the recommended standard finer pitch which were independently arrived at by myself and the committee of standards in our first automobile days of 1903.

For ring bolts nuts naturally the threads are somewhat special, but we still use a Whitworth form of thread, and nearly always of 16 threads per inch, but sometimes 20. There are a few exceptions - one is the sparking plug hole, which is a world standard metric, and if there are any other exceptions I should be pleased if they could be cancelled. I believe however that they only constitute an English standard pipe thread on one or two sizes of unions.

BY.{R.W. Bailey - Chief Engineer} will correct me if the above is inaccurate.

R.{Sir Henry Royce}
  
  


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