Piston rings can be easily and regularly examined for scuffing/burn evidence or ring breaks through the scavenge ports. Visual inspection of the size of the chamfer or rounding of the ring edges can also be used as a guide to wear, by comparing this with the size of the rounding on a new ring.
To enable this in-situ inspection, the engine turning gear is engaged to lower the piston just above bottom dead centre, allowing some of the ring set to be observed through the scavenge ports. Scuffing or burning is also a visual examination carried out using a torch. Checking for broken rings is carried out by pushing a piece of wood such as a broom handle (I used the shaft of a 2lb hammer) against each ring in turn. They should spring back to position against the liner; if they don't - a broken ring is a 99% certainty. All of the rings can be examined by raising or lowering the piston for access and inspection through the ports. A sketch of this method is shown below; please click on image to enlarge.
I sailed on a ship which had a Werkspoor 2-stroke main engine that broke piston rings on a regular basis, and I used the above method for inspection of the rings. The bottom part of the cylinder liner was in two halves and contained the scavenge ports. This section was easily split to reveal a full set of rings and the means to replace broken ones. I never did find the reason for the ring breaking, but suspected misalignment between the bottom section and the rest of the liner although these components were doweled, I wonder if they ever found the reason.
Enough of my rambling; a trait of an old Irish retired marine engineer, so back to the present. For a thorough ring inspection to be carried out, it is necessary to remove the piston from the liner, a sketch of this is shown below; please click on the image to enlarge.
Once the piston is on the inspection stool, the ring inspection can be carried out and the following conditions may be evident on the rings.
1. Good Condition
- Running surfaces will be bright.
- Rings can be moved freely within their grooves.
- Not unduly worn, (measurement required) and should exhibit a good lubrication supply.
- Edges will still have a relatively sharp chamfer-without any signs of indentations or burn mark.
2. Micro or Macro-Seizures/Scuffing
- Dull running surface.
- Scuffing-Appearance of vertical strip/sharp burn marks
3. Piston Ring Scratched
- Due to hard abrasive particle breaking from a badly machined ring or from particles entering cylinder via fuel.
4. Rings sticking in grooves
- Thick and hard deposits of carbon preventing the rings from moving freely in its grooves; caused by lack of sealing, i.e. combustion gas being blown past.
- Blackish appearance
- Black dry zones on upper part of liner wall.
As the ship’s engineer gains experience it will become second nature to examine the rings and spot any of the above conditions at the regular inspections or on re-assembly after a breakdown.