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Minimum squish clearance test

Discussion in 'Chainsaw' started by mweba, Nov 16, 2010.

  1. mweba

    mweba ArboristSite Sponsor

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    Well the other day I sat down in front of AS and started reading. Gathering info on minimal squish and maximum compression. The general consensus for safe work saw squish is .020. I have two saws (80+cc) currently running .0175 with no issues and decided to find the limit. Test saw is a ms660. Started with the squish at .016 and ran it for a tank with not an issue. Tore her back down and repeated with the squish less every time. Granted there are many variables in measuring but did the best I could.

    Any guess's how tight she got? Will say this, I had to slice my solder in half just to get the saw to turn over.
     
  2. parrisw

    parrisw Tree Freak

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    The problem is over time. With a squish too tight, I believe you can get erosion on the piston crown in the squish area. I find no need to go below .020, but that's just me. I can get 195psi on a 395 with .020, but also a popup.
     
  3. Stumpys Customs

    Stumpys Customs Stumpbroke

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    How tight is she?
     
  4. pops21

    pops21 Addicted to ArboristSite

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    I'm gonna guess .010"
     
  5. mtngun

    mtngun Addicted to ArboristSite

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    Besides bumping into the roof of the chamber, squish also affects flame speed. I'm not up to speed on flame speed, but if I understand it right, tighter squish increases flame speed, which can lead to detonation.

    I can't see pushing my luck for one or two extra psi.

    Admittedly, some saws like the 066 need all the help they can get in the compression department.
     
  6. blsnelling

    blsnelling Site Sponsor

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    My 066 is .017" using a factory gasket. I'm guessing you're down to .012".
     
  7. mweba

    mweba ArboristSite Sponsor

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    O I agree completely Will. It is more curiousity than anything. Also I just wanted to know how safe .020 was.

    Dang close
     
  8. mweba

    mweba ArboristSite Sponsor

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    Yep. Compression did not go up after .015. I believe running it hard would cause detonation. Had her up to 235ish

    .017 is just right IMO. Nothing really to gain after that.
     
  9. bcsizemo

    bcsizemo ArboristSite Member

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    It's highly unlikely I'll ever go that far into modding my saw, but just for curiosity sake, how are you measuring the squish?

    I assumed something like plasti-strip stuff for engine building?
     
  10. parrisw

    parrisw Tree Freak

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    What's the compression at with .017?
     
  11. blsnelling

    blsnelling Site Sponsor

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    I have no issues at all with that. On 50cc saws, I'll go as tight as .015"-.016". I know of a very good running 7900 that has a squish of .013".
     
  12. mweba

    mweba ArboristSite Sponsor

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    Tap dancing

    [​IMG]

    [​IMG]

    This was .011 at 235+- compression
     
  13. mweba

    mweba ArboristSite Sponsor

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    187 if I remember right
     
  14. chopperfreak2k1

    chopperfreak2k1 Addicted to ArboristSite

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    that was my guess too. great thread btw.
     
  15. parrisw

    parrisw Tree Freak

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    yikes!! Wtf!
     
  16. mweba

    mweba ArboristSite Sponsor

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    LOL
    It was a bad P&C so a good candidate for the test. Notice the compression band imprinted on the lower left and upper piston crown.
     
  17. parrisw

    parrisw Tree Freak

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    Yes I saw that, hence my post!
     
  18. mweba

    mweba ArboristSite Sponsor

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    This was a hot reading.

    [​IMG]
     
  19. mweba

    mweba ArboristSite Sponsor

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    Ha ya figured you would catch it. Funny enough I almost didn't before shaving the jug again and getting ready to assemble it. That would have been BAD.
     
  20. Stumpys Customs

    Stumpys Customs Stumpbroke

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    Did you notice any performance difference?
     

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