This is a picture of a scuffed liner.
What causes the presence of vertical scratches?
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2$\begingroup$ Because the pistons do not rotate they only go up and down. $\endgroup$– Solar MikeCommented Jun 23, 2018 at 20:16
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$\begingroup$ @SolarMike Are you sure? Piston rings rotate $\endgroup$– veronikaCommented Jun 23, 2018 at 20:56
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2$\begingroup$ When piston rings are fitted, assume 3 rings (2 compression & 1 scraper) they are fitted with the gaps at a 120 degree offset to avoid blow-by. As the temperature of the piston increases due to the combustion temperature, the gaps close up due to the expansion of the ring, The rings may rotate, but that rotation will be very slow - however the linear movement of the piston up and down will be much greater. The scoring is usually due to combustion particles getting trapped between the ring and the bore surface or the skirt of the piston coming into contact with the bore surface. $\endgroup$– Solar MikeCommented Jun 23, 2018 at 21:58
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$\begingroup$ @solarmike, why not post this as an answer? $\endgroup$– niels nielsenCommented Jun 24, 2018 at 6:05
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$\begingroup$ @nielsnielsen It is a completely wrong comment. I think that everybody is watching football $\endgroup$– veronikaCommented Jun 24, 2018 at 6:14
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1 Answer
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When piston rings are fitted, let us assume 3 rings (2 compression & 1 scraper) they are fitted with the gaps with a 120 degree offset to avoid blow-by (where combustion gasses leak past the rings - an issue that can happen when the gaps are all in line).
As the temperature of the piston increases due to the combustion temperature, the gaps close up due to the expansion of the ring, The rings may rotate, but that rotation will be very slow - however the linear movement of the piston up and down will be much greater.
The scoring can be due to combustion particles getting trapped between the ring and the bore surface or the skirt of the piston coming into contact with the bore surface. Once this has started then it just gets worse over time.
The gudgeon pin (aka "wrist-pin") in the piston is located fractionally off-centre to make sure that the piston is always tilted as it moves so that it does not "wobble" during the stroke, this wobbling can cause a noise called "piston-slap".
Edit: Another source of particles that can start scoring is the oil itself - if it is not changed regularly or other contaminants have entered the engine / sump...
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$\begingroup$ +1Thank you for the answer. I think that combustion particles is not the reason and that is why I posted the question. What I know is that high temperatures which are produced by scuffing change the cast iron structure to white cast iron which is brittle and after it breaks $\endgroup$– veronikaCommented Jun 24, 2018 at 7:27
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$\begingroup$ If your premise was true then all engines would suffer from that problem and I can tell you I have taken engines with 100,000 miles on them and stripped and re-built them with no evidence of scoring on the bores... Some engines may require a re-bore, others a light hone but that does depend on the damage. One had a severe deep score due to the gudgeon pin not being fitted correctly and it moved to the side contacting the bore. $\endgroup$ Commented Jun 24, 2018 at 7:31
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$\begingroup$ this phenomenon affects very long and slow ship engines i should have mentioned that in my question and appears when lubrication is problematic as it can be seen by the black patches on the photo $\endgroup$– veronikaCommented Jun 24, 2018 at 7:42
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$\begingroup$ it takes at least 14 hours for 5 people to replace the liner it is very difficult and expensive repair that is why I asked $\endgroup$– veronikaCommented Jun 24, 2018 at 7:44
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$\begingroup$ Then the source of the problem should be addressed i.e. fix the lubrication system. Ship engines usually have continuous acting oil conditioning or cleaning systems so that should be investigated. are there spray ports in the connecting rod? are they partially blocked? $\endgroup$ Commented Jun 24, 2018 at 7:49