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Help. Piston Selection Options.

Discussion in 'The Garage' started by colbystephens, Jul 15, 2005.

  1. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    So in beginning to look for new pistons for my 6.2L Diesel motor, I've come across an option which I am not familiar with. Apparently you can purchase pistons with coated or not coated skirts. What is the coating (the guys at the parts store didn't know), and what might be its advantage (it goes for $2 more per piston).

    The current pistons have 3 dishes in the tops of them. Is it important for me to keep with this type of piston, or is there another option which would benefit me in some way.

    Other than fit, skirt coating, and dish options, are there any other options which I should be considering in terms of replacement/improvement?

    Thanks!!

    Colby Stephens.
    www.web.pdx.edu/~colbys
     
  2. Mad-Dog

    Mad-Dog 1/2 ton status

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    Speed Pro and mahle have recently started offering coatings on the skirts of their pistons, these coatings reduce friction and free up some HP while increasing piston and bore longevity.
    It's well worth $2.oo a piston extra to have the coating.

    Any change in piston design from the factory configuration may alter compression, marine versions of the 6.2 run 19:1 CR vs the truck engine which has 18:1

    There are several ways to increase the 6.2's output from injector and turbo upgrades to methanol/water and propane injection.

    Speed costs money...how fast you wanna go?
     
  3. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    I suppose that makes sense about the friction reduction. Any idea what the coating is made of?

    Thanks!

    Colby Stephens.
     
  4. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    if i remember correctly, the coating on speed pro pistons is teflon. could be wrong though.
     
  5. Mad-Dog

    Mad-Dog 1/2 ton status

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    Unobtanium..... :haha:

    No clue, i think some type of moly based coating...good stuff tho.
     
  6. Kiwi John

    Kiwi John 1/2 ton status

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    Coated Pistons

    Hi Colby. Is this the same engine you were telling us about in another thread that had the "uncommmon bore size"problem?

    Anyhow, it is possible that the coating of the pistons could be what is known as a 'ceramic' coating. This is usually a white colour.
    Whatever it is, ceramic, teflon, unobtanium... it must be a modern improvement over the original.

    Re: The recesses in top of piston crowns. These are probably for valve opening clearance and injection spray pattern optimisation. You gotta have these if they were there originally i think.

    I remember hearing about a japanese-led ( I think) development in racing engines for oval shaped bores to reduce piston slap. i.e. the bores are oval in cross section shape & the pistons are oval to fit, as are the piston rings.
    This is obviously very high-tech. and only viable to Formula 1 Grand Prix type work.
     
  7. colbystephens

    colbystephens 1 ton status GMOTM Winner

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    Yes, I am still talking about that 6.2L with a need for a cure for the uncommon bore. Thanks for all your ideas. I appreciate it. I've been thinking tho about the coating on the skirts. Ideally, would a skirt ever touch the cylinder wall? As I've been taught, the only part of the piston which should touch the cylinder is the rings. If that is the case, there must be some other reason for the coating, right?


    Colby Stephens.
     
  8. Mad-Dog

    Mad-Dog 1/2 ton status

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    because the pin is offset the piston rocks in the bore and rubs the skirt,
    even more so as the piston descends into the bore and encounters the rod angle that is created as the crankshaft turns.
    90 degrees before and after bottom dead center the rod angle is greatest and this is where most piston scuffing takes place......hence coatings are applied to the skirt to reduce friction, heat and wear while simultaneously increasing torque and horsepower.
    If the pistons not rock in the bore and slid smootly up and down the cylinder walls then the bores would remain concentric (round) as they wore over time and minimal boring would be required when rebuilding a engine because the cylinder bore would be still concentric just worn from the tesion rings sliding up n down n up n down n up n down n up n down n up n down n up n down n.......................................................................................................
     
  9. Kiwi John

    Kiwi John 1/2 ton status

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    Skirts

    Hi Colby. Apologies for delay getting back to you. I agree with Mad-Dogs post. The coating, as I understand it, is to reduce friction and aid heat dissipation. The skirt of the piston rubs the bore as it travels up & down. One side rubs un the way up and other side rubs on the way down. Oil is the stuff that minimises the wear from rubbing. Close tolerances from machining stops too much deflectionof the piston during travel. The straighter you can keep the piston in the bore, the better the rings will seal = better compresion = better combustion > less bypass into crankcase > longer oil life etc.
    Notice how older/early engines had really long skirts on their pistons. Lately u see pistons with cut-aways at the sides either end of the gudgeon-pin but they still have a remnant of the skirt to keep piston straight. (The cut-aways are for weight reduction to allow higher revving, mainly of petrol engines.)
    Good Luck with the re-bore. Don't skimp on the basics. :)
     

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