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).
Fitting Perfect Circle piston rings and drilling holes to address overoiling issues.
| Identifier | ExFiles\Box 16\1\ Scan115 | |
| Date | 30th August 1930 guessed | |
| - 2 - The "stunt" of drilling holes behind the lower ring is not a new one - it has been used in this country and in Equrope for many years past and it is a wonder to me that Rolls Royce have never used it. I understand Mr. Steele has cured overoiling of 20-HP engines in this same way. Re Correct Fitting of Rings - Just about the same time that the Perfect Circle Ring was being standardized, we discovered that the shop were fitting rings without any clearance at all instead of .010-.015 as specified. We were unable to account for this but we made them revert at once to standard figures with an immediate and marked reduction in the internal friction and increase in freedom of the engine and general cleanliness of oil after test, and at the same time an improvement in slipper action. The reason for this being that the rings get hotter than the cylinders and take up a certain amount of clearance by expansion, and if there is no clearance, or insufficient clearance to start with they are forced hard against the cylinder walls with excessive friction and wear. I have seen piston rings worn out in this way before the engine passed off test. I do not know whether this change had any effect on oil consumption. Now coming to the PII chassis - England are now using a lower ring similar in construction to the Perfect Circle that we call for (the only difference being a large number of drilled holes where we have slots) but do not drill the piston in the bottom groove. Your letter gives the first suggestion that we have met trouble with overoiling on PII chassis and I am not surprised as I have contended for some time that it was not the Perfect Circle ring that gave us the improvement we got but rather the drilling of the holes in the piston back of the ring, and would be particularly interested to hear the result of this on Mr. Ainsworth's engine, which was just beginning to overoil. Possibly I am to blame to a certain extent for not having urged this matter more, but as our troubles in this respect seemed to be dying out, I am afraid the matter was overlooked. To Sum Up. I have not reported on this to OX because I was not satisfied in my own mind exactly how the results were arrived at and am still awaiting information as to the result of drilled holes in the piston behind our standard ring. If we tell him that Perfect Circle rings give us a wonderful improvement - which he already knows - it is not saying very much since England are already using a very similar ring, but if we can say we get similar results just by drilling a row of holes in the piston behind the bottom ring - using our standard rings - then we are telling them something - and if we can go further than this and tell them we have cured overoiling on PIIs by just drilling this same row of holes in their piston with no other change - this would be really valuable information - but I do not feel justified in doing this yet on the information I have available. M Btn. | ||