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New cylinder bad compression

Discussion in 'Chainsaw' started by Pim, Nov 14, 2017 at 11:48 AM.

  1. Pim

    Pim ArboristSite Lurker

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    Hello all!


    I'm new to the forum.
    I got a Stihl MS181 with seized piston. First tried to revive cylinder but no luck
    As these are very cheap saws I decided to buy a cheap Chinese cylinder (including piston etc. ) for $20 on eBay.
    After rebuilding it has low compression about 90psi.
    I starts, but has very low power, even for a MS181 J
    Can I do something to fix? Try to hone it?
    Or was it a waste of money?
     
  2. backhoelover

    backhoelover 2 cycle carb master, don't believe me just ask!!!!

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    Cant Hone cylinder they have a coating. Need to pull cylinder and ring off piston push the ring into cylinder and measure your end gap
     
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  3. earlthegoat2

    earlthegoat2 ArboristSite Operative

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    Might want to measure your piston too.
     
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  4. Ozhoo

    Ozhoo Addicted to ArboristSite

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    Welcome to AS. Have you taken a look through the exhaust port? You wouldn't be the first to trash a new cylinder.
     
  5. brandonstc6

    brandonstc6 Addicted to ArboristSite

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    I would say the supplied rings are no good and possibly the piston is too short and the squish is too much.


    Sent from my iPhone using Tapatalk
     
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  6. cus_deluxe

    cus_deluxe Thats what she said.

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    Most likely IMO. Ive seen/installed aftermarket kits with squish in excess of .060. Would run decent but with low compression and power. Tho i would first check to make sure stuff isnt scored or anything else obvious.
     
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  7. HarleyT

    HarleyT Addicted to ArboristSite

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    Here is a TB on different cylinder assemblies, may be a factor, I don't know.
     

    Attached Files:

  8. Pim

    Pim ArboristSite Lurker

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    Thanks for the quick response.
    I checked the piston through the exhaust, no scoring looks perfect.
    I will disassemble the engine tomorrow or day after and report back.
     
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  9. Duce

    Duce ArboristSite Operative

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    Did you check squish yet?
     
  10. SAWMIKAZE

    SAWMIKAZE Addicted to ArboristSite

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    Stens kit on a ms260 was at 57 on squish and was pretty pathetic in the power dept...pretty crappy rings too.
     
  11. brandonstc6

    brandonstc6 Addicted to ArboristSite

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    I’ve had an aftermarket 028 super piston in a oem cylinder with .060 squish and 120 psi compression. I took it out and used a Tecomec (made in Italy) aftermarket piston and the compression was 155 psi. I didn’t check the squish with the tecomec piston.


    Sent from my iPhone using Tapatalk
     
  12. Thommo

    Thommo ArboristSite Operative

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    It is possible that the conrod bent when it originally seized. This will increase the squish so you have low compression.
     
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  13. alexcagle

    alexcagle Cutoff Saw Specialist

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    I wouldn't be at all suprised if an MS171 37mm ring set got stuck in an MS181 38mm kit. lol
    It's easy to catch on a 2mm jump during an install, but a 1mm might slip by.
     
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  14. Pim

    Pim ArboristSite Lurker

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    The squish I measured is 1,4mm (0.055”). Is this too much?
    I don’t have tools to measure the gap when the ring is in the cylinder, so made a picture.
    The piston ring is 1,5mm x 1,2mm, so the gap is about 0,4mm derived from the picture.
    The piston is 37.85mm
    Cylinder measures 37.9mm (although difficult to measure)

    Knipsel.JPG
     
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  15. brandonstc6

    brandonstc6 Addicted to ArboristSite

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    I would say the squish is too much. Could the cylinder be out of round?


    Sent from my iPhone using Tapatalk
     
  16. cus_deluxe

    cus_deluxe Thats what she said.

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    Seems like that squish is pretty typical of AM top ends. Ring end gap seems pretty close, close prob be a little tighter, but the excessive squish will, i think, hurt performance more. As said above, you may be able to find a different piston that has more compression height.
     
  17. rupedoggy

    rupedoggy Addicted to ArboristSite

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    Put that cylinder on your belt sander. Keep the bottom square and bring the squish down to .020. Your saw will run!
     
  18. Pim

    Pim ArboristSite Lurker

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    Nice idea but it will not work for this type of cylinder. Sanding the bottom will not lower the seating of the bearings so this will have no effect on the squish.
     
  19. Thommo

    Thommo ArboristSite Operative

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    You can sand down the bottom of the cylinder, but you need to grind out the bearing pockets by the same amount.
     
  20. brandonstc6

    brandonstc6 Addicted to ArboristSite

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    I would compare the dimensions of the old piston and the new one and see what you come up with.


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