Auto piston skirt ?

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I have noticed several car manufacturers coat the lower part of the engine piston skirt with a coating of some kind and they say to reduce wear? I guess I am missing something as I was always under the impression that it was the piston rings on the piston that are the only thing that contacts the cylindar wall? Why would the lower part of the piston need a coating?
 
its all part of switching to a lower friction engine setup.

Less reciprocating mass, less friction losses etc.
 
Where working HTHS could be much less than 3 cSt, and, the skirt lubrication will be at boundary conditions, the moly pad is an anti scuffing friction reducer. It CAN wear against the cyl walls.
 
Generally truck pistons are 4-ring with a lower oil control ring. They are the only pistons that I know that have low skirt contact. All 3-ring (and less) pistons will be against the cylinder wall due to rod angle thrust. So it's all about material comparability (aluminum alloy/iron) and the oil doing its job ...

Piston skirt/bore wear is one of the things that causes many of us to want to keep the HTHS values up there ... It's one of the thick(er) oil drivers in an oil selection. Generally at normal piston to wall clearances, SAE30 is the desired oil (or equal). 40 if loading it hard (towing). 50 if racing with higher heat/RPM loads.

The guys that are running 20 grades are generally just commuting, or family putting, etc. They all say they get 200,000 w/o issue and tout that as evidence that is the way to go. I'll bet a good lunch that most of them will trade the car in w/o ever having been to redline or towing up the Cascades, the Sierras, or the Rockies ...

Landscapers in Florida say they get a bazzilion miles out of their F-150's towing and in the heat. But Florida is about flat
laugh.gif
With decent tire pressures, how much work is that ...
 
I'm not a thin oil proponent at all, but my engine returned amazing results using 5w20, towing 3 tons over rated capacity on Dino.

Florida may not have mountains, but that could hardly be considered an easy tow on the drive train.
 
Originally Posted By: Camprunner
I guess I am missing something as I was always under the impression that it was the piston rings on the piston that are the only thing that contacts the cylindar wall? Why would the lower part of the piston need a coating?


The lower part of the piston needs a coating because it contacts the cylinder wall. A simple computer animation may show a completely vertical cylinder and piston that doesn't rock back and forth, but the piston skirt drags against the cylinder walls all the time. Piston skirt wear:



https://www.youtube.com/watch?v=6uK7i4NHuEA
 
Yes, and I've seen lots of engines with that coating worn mostly off the piston skirts.

Edit: That still ran fine.
 
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I think the coating keeps the engine quiet and possibly better performing when you're test-driving a new car. Nobody cares what happens with the coating after that. Hyundai uses graphite on some piston skirts.
 
Originally Posted By: Hokiefyd

The lower part of the piston needs a coating because it contacts the cylinder wall.


It doesn't need one, but they have been added in recent years to reduce friction and scuffing. GM added a coating to the LSx pistons to help get rid of the piston slap noise the engines were known for for example. It didn't mean that the engines that lacked the coating were going to fail early, but they were loud when cold (slap).
 
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