when looking at modern coated skirt pistons, do you guys consider the coating "sacrificial" to a degree and therefore take extra care to not run the piston too "loose" when sizing the hole. Sometimes you have little windows on the skirt to MIC an uncoated portion of the piston, other times you do not.
even if they say the coating isn't sacrificial, how many times have you guys torn down a high mileage (50k) high performance engine with the coating still in-tact?
I am not speaking about L2L which is indeed a sacrificial coating that runs itself into the ideal clearance.
coated skirts (teflon?)
Moderator: Team
Re: coated skirts (teflon?)
As you have pointed out, some coatings are sacrificial and others wear resistant [even though they do show wear eventually] As for gauge point, even if a point is not identified on the piston the manufacturers fitting instructions will indicate one.
We also have abradable coatings [as your quoting of LTL]that when the piston is installed they are fitted size for size, that is, the piston gauge point is the same size as the finished bore, on some of these pistons below the gauge point a slightly larger o/d can be identified which means the piston is actually installed with slight push in the bore, the coating is sufficiently thick that after the engine has made it's own bore clearance, the wear stabilizes to virtually non existent after the bedding in process. What is interesting with size to size is that the final wear clearance is almost the same as the fitted clearance of a non abradable piston. [although the non abradable is usually tin coated].
As for service life of coatings, i have seen them with upward of 100K km on them and the coating shows pretty much hazing to a visual show of the pistons diamond turn finish,but no complete loss as such, i would measure skirt collapse more so than wear, ring lands also get more of a hiding than the skirts,[we see a lot of coated ring lands now] i also find latter pistons show little wear in the gudgeon bosses after extended use, i think part of this is the pins run far less clearance in the piston and rod than earlier versions of one engine that ran .001" to .0015" that is now .0002" ,there may have been a pounding effect with the larger clearance.
Interestingly, what were considered cheaper after market replacement pistons now come with skirt coating and land coatings, economies in production can be seen by cast in oil drains with vents for the upper oil ring provided by deepening the drain slot, cast valve reliefs are also a more common sight as well. Precision of dimensions is also consistent.
Cheers.
We also have abradable coatings [as your quoting of LTL]that when the piston is installed they are fitted size for size, that is, the piston gauge point is the same size as the finished bore, on some of these pistons below the gauge point a slightly larger o/d can be identified which means the piston is actually installed with slight push in the bore, the coating is sufficiently thick that after the engine has made it's own bore clearance, the wear stabilizes to virtually non existent after the bedding in process. What is interesting with size to size is that the final wear clearance is almost the same as the fitted clearance of a non abradable piston. [although the non abradable is usually tin coated].
As for service life of coatings, i have seen them with upward of 100K km on them and the coating shows pretty much hazing to a visual show of the pistons diamond turn finish,but no complete loss as such, i would measure skirt collapse more so than wear, ring lands also get more of a hiding than the skirts,[we see a lot of coated ring lands now] i also find latter pistons show little wear in the gudgeon bosses after extended use, i think part of this is the pins run far less clearance in the piston and rod than earlier versions of one engine that ran .001" to .0015" that is now .0002" ,there may have been a pounding effect with the larger clearance.
Interestingly, what were considered cheaper after market replacement pistons now come with skirt coating and land coatings, economies in production can be seen by cast in oil drains with vents for the upper oil ring provided by deepening the drain slot, cast valve reliefs are also a more common sight as well. Precision of dimensions is also consistent.
Cheers.
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