Wednesday 20 November 2013

On modern piston rings in old Norton bores

-SG- Juris in Latvia has raised some interesting questions on this subject with this old Norton bore and we invite those who have practical experience of such matters to let us have their views. Juris says:

"I am working on several Norton engines at the moment, two CS1 models of 1930 and 1938 and one Model 18 of 1930. As you know these have different lubrication systems with the 1930 Model 18 being of "total loss" and the other two of the "dry sump" type. I have got specially made forged higher compression aluminium pistons for all three and the pistons have been machined to use modern VW rings 1.75/2/3 mm. I have used modern technology Flex-Hone instruments after honing the cylinders instead of the old fashioned polishing with "Brasso" liquid metal polish as today's automotive engineering technology advises the use of a cross-hatch pattern of tiny uniform scratches on the cylinder surface to provide oil retention, better compression and less oil consumption. The modern automotive rings have higher precision and are also usually honed on the outside diameter for perfect roundness. The top ring is chromium plated as is the oil ring. 

The cause of my concern is the use of the modern (high pressure) oil ring in an older engine with total loss lubrication, like the Model 18 of 1930. The ring has a coil spring which is made to fit on the inside of the ring and the pressure outwards is very substantial, compared to old-fashioned solid cast iron rings. I have successfully used forged pistons with modern automotive rings in my 1913 "total loss" REX/JAP 6HP (770cc) engine for around 4000 kilometres. This is lubricated by vegetable oil and no oil rings were fitted as this type of engine had been designed to burn a certain amount of oil. I am now wondering whether I should:

  1. Discard the oil ring and use the piston in my 1930 Mod 18 with only 2 compression rings, or perhaps fit a suitable width 3rd compression ring in the oil ring groove.
  2. Discard the inner coil spring under the oil ring and use the oil ring without the inner spring.
  3. Shorten the inner coil spring of the oil ring to allow for less pressure on the cylinder walls to save them from wear and to let through more oil.

I am going to use the modern oil rings with reduced spring pressure in my dry sump OHC engines. Needless to say the modern mineral oils available seem to be far superior to what was available to the rider back in 1930 or even 1938, with all those antiwear and antioxidant additives etc."

My personal experience of such matters is way out of date so I sent a copy of Juris' message to Chris in Scotland whose business involves refurbishing Vintage and Post Vintage engines and has much experience in such matters.  Here's what Chris recommends:

  1. In 'total loss' engines, use three plain compression rings as per the original design.
  2. In relatively low flow dry sump engines - for instance Model 25s or Moore CS1s - use two compression rings and one stepped oil control ring.
  3. In higher flow dry sump systems such as the CS1s Juris is working on, use a normal oil control ring of orthodox type. I take the view that it is beneficial to increase slightly the diameter of the holes behind or just below the oil control ring.
  4. If you are using a modern piston then it's not a good idea to remove the spring behind the oil control ring.

Although he has no connection with the firm, Chris gets his rings from an outfit called Cox and Turner - which specialises in rings of all types. They can be contacted as follows: