Piston Rings

[dave bushell]

Does anyone know what the diameter of a piston ring is relative to the cylinder bore? Knowing that would allow me to identify some piston rings in my possession that are without their packaging and as yet, not been ground on their ends for the ring stops.
Dave

[efr215]

The problem is that in their unrestrained state piston rings ‘aint round!

This is because the ring is split and because of that a truly round ring would not exert an equal pressure on the bore when in place. To counter this rings are made to a shape, when relaxed, approximating to that of a halved apple, a shape that evens out the radial pressure all around the bore when fitted.

The simple answer would be to make several pieces of sheet steel with holes in them matching the standard bore and the usual oversizes. The rings could then be inserted, largest hole first, until the best fit is found when held up to a strong light.

[dave bushell]

HI efr215
Unfortunately, your simple solution is not so simple if you don’t have the facilities to produce such test plates. To find someone to make them probably wouldn’t too easy either, especially economically.
Dave

[scott596]

David first of all piston rings are an accurately made item, you say that they have not been ground for the ring stops which makes it easier to check them. Lets suppose that you have a Scott barrel of known size IE,long stroke plus .040″=74mm if you put the correct size rings into the bore you would probably have .012″ end gap if you put plus .020 rings into the bore IE 73.5mm you would have about .070″ end gap and if you put plus .060″ rings into the bore IE 74.5mm the ends would overlap each other by about .060″. So although they might look similar the circumference is quite different. Alan Noakes. scottworkshop@hotmail.co.uk David I have sent you a PM.

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