Piston Skirt Coating

Posted by bastarddsm, Apr 19, 2018

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  1. bastarddsm

    bastarddsm Supporting Vendor

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    Hello Everyone,
    I assume everyone these days know the benefits of coating technologies, but just in case I'll give a brief description. Piston skirt coatings are typically a Molybdenum Di-sulfide based dry film lubricant. Moly is an extremely high pressure lubricant that survives when many other lubricants fail. Coating pistons skirts helps reduce friction, and reduce the chances of galling due to extreme loads. This is essential because even a modest 4G approaches 100hp/cyl, a point that would be considered "extreme race" in most of the rest of the world.

    So a few years ago I built a motor with these really nice JE pistons that had coated skirts. I ended up hurting a piston and needing to get it re-coated. After many phone calls and much time I decided it was just best to do it myself. So I did the research, procured the materials, and coated a set of pistons. After 3 seasons of successful testing, I can finally offer this service. Prices start at $100 for a set of 4.

    Here's a picture of the finished coating,

    new skirt coating.jpg

    Here is the set of pistons after 3 seasons of abuse - Keep in mind this is 600+whp and gets taken to 9500 every pass, and I go racing a lot. This thing has had many 50psi excursions, and has never trapped less than 130, and has been 140's for the last 2 seasons.
    used piston skirt.jpg used piston skirt2.jpg
     

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  2. 1990TSIAWDTALON

    1990TSIAWDTALON Moderator

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    Thanks Kurt, for a new service for the community. Seems reasonably priced to me!
    Your car speaks for your work, and it is a mean machine!
    Marty
     
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  3. Tanro

    Tanro Proven Member

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    Hot sh** dude. That is out f***ing standing work. And you can get this done for us at 25$ A piston? That is just excellent. If/when I get around to building a shortblock I will be sending you a set.

    Those pistons look absolutely amazing considering the stress load you've put them through.
     
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  4. bastarddsm

    bastarddsm Supporting Vendor

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    Yes well $100 for a set of 4. Singles the price'll go up a bit - setup is most of the work so it's easier to do groups. And to be fair most others do it for the same price, but usually turn around time and not loosing your parts is an issue.

    Also I don't have a thermal barrier type coating, I could probably figure it out, but I just don't believe in them.
     
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  5. ViciousLord

    ViciousLord Proven Member

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    Could this coating be used for other areas in the block and or head? Just thinking outside the box.
     
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  6. bastarddsm

    bastarddsm Supporting Vendor

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    Yes, you could do like bearings, inside of the piston pin bore, ect. I've not done anything outside of piston skirts, so it would be an experiment.

    I think that maybe 2g thrust bearings would be a place to try it. Rod/main bearings don't need much since they have a constant supply of fresh oil and ride on a hydrodynamic wedge. They ride on a wave of oil and there is never metal to metal. Piston skirts and thrust bearings don't work the same way and there is much more likelihood for metal to metal.
     
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  7. m_0ney_pit

    m_0ney_pit Supporting Member

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    Will you coat stock style NPR type piston skirts?
     
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  8. bastarddsm

    bastarddsm Supporting Vendor

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    Yes, but they need to be off the rods.
     
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  9. m_0ney_pit

    m_0ney_pit Supporting Member

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    Do you grit blast the skirt before coating?
     
    My DSM:
    1998 Eagle Talon TSi AWD

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    1999

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    16g   manual
    1999 Mitsubishi Eclipse GST

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  10. bastarddsm

    bastarddsm Supporting Vendor

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    Yes, that's why they need to be off the rods. The pin bores and ring lands are masked off.
     
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  11. m_0ney_pit

    m_0ney_pit Supporting Member

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    so the coaters typically brag about not having to change clearances after blasting and coating. Personally I have seen mill finished aluminum (forged probably swells less) swell up to 1 mil after blasting with oxide so both sides plus maybe a half mil of coating you beef up the piston diameter about 2.5 mil after the process. What's your take on doing the machine work and then coating or coating and then machine work?
     
    My DSM:
    1998 Eagle Talon TSi AWD

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    1999 Mitsubishi Eclipse GST

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  12. Jon Lane

    Jon Lane Proven Member

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    This is fantastic work, OP. I know that while I'll never race, your posts are still always worth studying. You know what you're doing and the content of your comments is educational.

    However I've got an experience with split 2G thrust bearings in a NOS 7-bolt build to relate. This particular new crank and block didn't like aftermarket thrust bearings. A set of Kings and two sets of coated ACLs were all too thick - giving just slightly under .002" end play - and only the OEMs were thin and consistent enough to fit.

    Measuring the aftermarket sets showed that they were all less flat than the OEMs too. This lead to mixing and matching one good pair out of the eight pieces. It also led to ever so slightly emerying the final pair in order to flatten them.

    In none of these cases was the coating the apparent problem - the aftermarket shells themselves were thicker and less flat. In the case where the end play is already on the low end of the scale, and in a touchy application where flatness is key to a successful oil film, it could be - based on this limited experience - that a 2g split thrust coating service could benefit from truing the shells first.

    Is that possible or is this just an unusual experience?
     
  13. bastarddsm

    bastarddsm Supporting Vendor

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    That's interesting and something I've not considered. I've always coated first, and then machine. It typically measures about a .0005" coating thickness, so finished size is .001" bigger. I take about .0005" off the clearance for for the coating. IE, my target clearance is .0045, so I set the actual clearance to .004". I just did a new set of JE's out of the box they measured 3.3625", after coating they measured 3.3635". The whole thing gets tricky since aluminum grows .0005" per 10*f and you and you need to preheat the pistons before you spray them, so you are working with a range of temperatures.
    I never measured one after blasting, I'll check that out next time. The blasting is fairly soft -150grit aluminum oxide at like 50psi.


    Thanks Man!

    I totally agree. The coating process is additive and defiantly would add to the thickness. I've not had any issues with thrust bearings, but I do know it's common to hold a pair together with a hose clamp and flat sand them on a piece of 4-600 wet/dry on a piece of glass. The nice thing is the coating would wear from the high spots, and you should end up with a nice flat surface. I think this would require quite a bit of experimentation to get it right though. Thinking about this gave me a great idea for a tool to square up the thrust bearings in the block.
     
    My DSM:
    1991 Eagle Talon N/T

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  14. m_0ney_pit

    m_0ney_pit Supporting Member

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    Valve stems, I coated a set of 6t valves used tin foils to mask the seat and seal top/bottom when I put new rings in the 6 bolt last summer.
     
    My DSM:
    1998 Eagle Talon TSi AWD

    Street Build

    20g   automatic
    1999

    Street Build

    16g   manual
    1999 Mitsubishi Eclipse GST

    Street Build

    14b   manual
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