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Slug and jug clearance

Discussion in 'Chainsaws' started by countryhog, Oct 9, 2018.

  1. countryhog

    countryhog BTDT

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    put new rings in craftsman 3.7 with LOW compression. All i can get now is 100 psi; bout what it was before replacing. Cant hardly start it but when i do compression is so low it wont idle and wont stay running at WOT. Dud all the normal stuff: carb, lines, filter, etc. even replaced coil with known good one since spark was nowhere hot enough. got couple of suspicions
    Even thiugh both the slug and jug look good, no scoring, im wondering if clearance between them is too much. I checked new ring gap at bottom of ports and they were spot on.
    Or - not sure if the jug is chrome or the piston. Either way, will that affect ring seating? Rings are ductile and are from greece
    Gettin really frustrated here so all help is appreciated
     
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  2. mortalitool

    mortalitool aka Nick Stoeckel

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    Do you have another compression tester to try? Could be that your comp tester is faulty.

    Quick question: Does your comp tester have a schrader valve on it?
     
  3. countryhog

    countryhog BTDT

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    Just the one. Got it from a reputable site member who swore by it. Yes it has a schraeder valve
     
  4. Bigmac

    Bigmac Pinnacle OPE Member

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    dose it “feel” low? What is the squish? Running a gasket? Have you tested the compression gage on one of your other “good” saws for a kind of baseline?
     
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  5. DrewUth

    DrewUth Too weird to live, too rare to die.

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    To answer your question about rings- yes, rings meant for an unlined cylinder and a lined (plated) cylinder are different and will not seat properly if installed in the wrong type of cylinder.
     
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  6. countryhog

    countryhog BTDT

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    Has a distinct pressure feel at tdc. Running gasket. Didnt measure squish cause put it back to stock. Compression gage gives plausible. Results for known runners.
     
  7. countryhog

    countryhog BTDT

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    Thanks. Now i get to research that
     
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  8. Chainsaw Jim

    Chainsaw Jim #fartedinthebandwagon

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    Wrong rings or not it should blow harder than 100. I'm thinking this saw was possibly a dud from the factory with too much squish area... maybe the wrong gasket was used. I'm doubting you overlooked bad spark plug threads, so checking squish to see if there is an immediate improvement might be a good start.
     
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  9. smokey7

    smokey7 Pinnacle OPE Member

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    What is the p&c clearance what is the squish? Also im fairly sure that is actually a Roper saw rebranded to craftsman and poulan. As such i dont think they had the plane bore with a plated piston. So as long as you bought rings by application not by size it shouldnt be a issue. Lastly if you got rings for a bare bore ie chrome faced or similar material will or can destroy the plating in a good bore rather quickly.
     
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  10. countryhog

    countryhog BTDT

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    Really? How did you see this saw without me not knowing about it. Most definitely NOT a roper. Roper branded to craftsman as did poulan. Roper did not brand to poulan
    Thanks for the ring tip
     
    Last edited: Oct 10, 2018
  11. smokey7

    smokey7 Pinnacle OPE Member

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    My mistake. I read 3.7 and didnt think further on it. My apologies
     
  12. countryhog

    countryhog BTDT

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    Correction. Ordered rings directly from sears parts plus from the parts diagram on their web site. So, rings are the right ones or sears shipping screwed up
     
  13. countryhog

    countryhog BTDT

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    Finally got around to checking squish.
    0.10”
    WHAT?
    Think i may have found the problem.
    Tear down in order. Believe my base gasket material to be the issue. We’ll see
     
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  14. Fruecrue

    Fruecrue I got wood

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    That’s a damn cavern!
     
  15. Bigmac

    Bigmac Pinnacle OPE Member

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    That’s great you found it! Checking squish is always a good idea, It is amazing how much variance there is, on ms660 I have was .019 squish stock and another was .035 stock
     
  16. countryhog

    countryhog BTDT

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    Reinstalled jug without gasket. Squish is now 0.05 (actually was 0.08 before; measured the solder wrong). Compression is now 130 psi. Still cant get this sucker to start. Even with a prime. Gonna try different carb. AGAIN. gonna make sure flywheel didnt shear key. After that im out of ideas. This thing really taking too much time. May end up in christmas thread
     
  17. Bigmac

    Bigmac Pinnacle OPE Member

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    That’s a nice bump in comp, it’s still a lot of squish, getting it down to .025 would probably give you another 25psi of compression! It must have been a poorly machined cylinder
     
  18. Stump Shot

    Stump Shot Old *pretty boy dipstick of Quality

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    130PSI while not optimal, is enough for the engine to run. It should at least "Pop" with a prime and usually run for a couple seconds even with a completely dead carburetor. I would look into other areas such as a primary crankcase pressure test and a spark test.
     
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