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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).
Design discussion on split aluminium pistons for the 40/50 HP chassis, focusing on cone shape and ring placement.

Identifier  WestWitteringFiles\D\February1920\  Scan53
Date  20th February 1920
  
To D.H. from E.{Mr Elliott - Chief Engineer}
COPY to Col.
" " CK.{Mr Clark} X.3924.
ORIGINAL.
RE SPLIT PISTON IN ALUMINIUM FOR 40/50 H.P. CHASSIS.

With reference to drawing No. N.3510, would you kindly note that Mr. Royce agrees to the use of a slot .050" wide for the longitudinal split only. He would like the taper top to be of the dimensions that have already been used for these pistons which, so far as can be remembered, give a cone .400" high.

It would be necessary to raise the shoulder of the piston to keep the same compression with this .400" high cone. Mr. Royce considers the .600" cone makes too sharp a point on the piston top.

It is desired further that the metal behind the piston rings shall be tapered in thickness so that the section falls in area as the heat flows away.

Referring to the top-most band of the piston which is finished at 4.46" diameter, Mr. Royce does not quite understand how this is intended to work. He thinks it is a place where the carbon would tend to be deposited rather than otherwise. It should be demonstrated whether this reduction in diameter is an advantage or not.

If it is necessary to bring the hot gases down to the point shewn, then it is thought that the piston would do better if the rings were half their present distance from the top, that is to say, if there is no virtue in having the rings low down, it would be better to put them higher, making a lighter piston and one with more wear in it.

E.{Mr Elliott - Chief Engineer}
  
  


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