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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 The Light Production Co. Ltd. discussing piston clearances and temperature calculations for a new set of pistons.

Identifier  ExFiles\Box 148\3\  scan0229
Date  5th June 1937
  
Experimental Dept.
5th June 1937

Rm{William Robotham - Chief Engineer}/Mths.{Reg Matthews}3/AP. 1256

The Light Production Co.Ltd.,
60/66 Rochester Row,
Westminster,
LONDON S.W.1.

For the attention of Mr.C.H.Brooks.

Dear Sirs,

This is to confirm our conversation of the 1st. inst., in which we asked you to supply one set of pistons with the heat break sawcut in the scraper rings groove suitable for bores .010 oversize and fitted with 32 lbs/sq.in. scraper rings. We have instructed our Order Office to issue an official order to cover these pistons.

With reference to the piston which failed on test we find that the clearance between the connecting rod small end and the gudgeon pin bosses can be from .005 to .020. Should the clearance be at the low end of the scale I calculate that the actual running end clearance will be about .0005", so that it seems quite possible that the small end bush could under certain circumstances be exerting quite an appreciable pressure on the bosses. I have assumed from my calculations the temperature of the gudgeon bosses to be 200°C. and the temperature of the small end to be 130°C. You might let us know what you think the actual working clearance would be, or whether 200°C. is a reasonable figure for the temperature of the bosses.

We intend to run a set of pistons with varying clearances to prove the point and will let you know the results in due course.

Yours faithfully,

FOR ROLLS-ROYCE LTD.
  
  


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