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Which Piston would you use? (Stihl 026 Pro)

Discussion in 'Chainsaw' started by davefr, Aug 11, 2018.

  1. davefr

    davefr Addicted to ArboristSite

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    Guys,
    I bought this "project saw" super cheap at a G-sale. It's a like new Stihl 026 PRO. Someone started with a top end rebuild and quit in the middle of the project. I don't think there's a single problem with the top end. However I did find that the impulse hose was not connected.

    I'm trying to decide on which piston to use. On the left is the original Stihl piston with extremely light scoring on the exhaust side. I can't even detect the scoring with my finger nail. On the left is an new unbranded aftermarket piston.

    I'm kind of leaning towards re-using the OEM piston despite the extremely light scoring. The cylinder looks perfect as does the intake side of the original piston. What say you??? TIA.

    P8110002.jpg P8110006.jpg P1090610.jpg
     
  2. r black

    r black ArboristSite Guru

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    that saw is very clean..... think I would spring for a new meteor on eBay for $34 or new factory stihl piston is around $50..I would not use that aftermarket piston in the pictures, but you could with a new set of caber rings ...appears someone already sanded on the piston that has scoring ...
     
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  3. rupedoggy

    rupedoggy Addicted to ArboristSite

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    Clean it up with scotch brite pad and reuse the Stihl. Mike
     
  4. MontanaResident

    MontanaResident Addicted to ArboristSite

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    Or 1000+ grit sandpaper.

     
  5. Chainsaw Jim

    Chainsaw Jim Addicted to ArboristSite

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    I'd recommend a new oem piston, gasket kit with seals, carb kit, fuel line/filter, and you'll be set for a long time to come.
     
  6. Mad Professor

    Mad Professor Addicted to ArboristSite

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    How is skirt clearance on the used OEM? Be careful there is no grit embedded in it from sanding. They are about $60 (44mm) now. Meteors $35-40
     
  7. davefr

    davefr Addicted to ArboristSite

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    That's an interesting question. The OEM piston is actually slightly tapered. It starts at 1.753" by the rings and tapers down to 1.744" at the skirt. The new aftermarket piston is a consistent O.D. of 1.760"

    The OEM piston w/o rings installed can be rocked noticeably in the cylinder. The new aftermarket piston can be rocked but it's barely perceptible.

    The aftermarket piston seems to be a better fit but the OEM piston does not look worn except for the light scoring.

    Now I'm leaning towards using the new aftermarket piston. I don't know the brand but the new piston rings are Caber from Italy.

    Do Stihl OEM pistons have more skirt clearance by design for thermal expansion contraction reasons?

    New piston on the top image. Stihl piston on the bottom.

    P8120010.jpg P8120008.jpg
     
  8. rupedoggy

    rupedoggy Addicted to ArboristSite

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    analysis paralysis but carry on.
     
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  9. wcorey

    wcorey ArboristSite Guru

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    14 thou clearance at the skirt is a well worn out piston...
     
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  10. Mad Professor

    Mad Professor Addicted to ArboristSite

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    OEM piston (and other quality pistons) will be tapered with the skirt being larger. Your piston has a lot of wear if the numbers you posted are correct, the taper should be in the opposite direction. Check the wear in several places because it will vary.

    The taper I've measured on NOS OEM 038 assys were 0.005-0.006" with the skirt larger.

    If your taper is correct then that might explain the sloppy skirt clearance (0.0145"), new should be ~ 0.002". That piston is well worn.

    What is the source of the aftermarket piston? If it has no taper it's a cheapie. How do you know the rings are cabers , but nothing about piston?

    If you want things to last I'd use OEM or Meteor piston and OEM gasket set/seals. Look over very closely or replace, impulse and fuel lines and intake boot.

    What carb is on it? Does it have both Hi and Lo screws?
     
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  11. davefr

    davefr Addicted to ArboristSite

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    Thanks. This was very helpful.

    Actually both pistons are slightly smaller O.D. at the dome and gain about .002-.003" in O.D. about half way down. However the original piston's O.D. falls off sharply after about 2/3 of the way down and then drops .010" at the skirt. The new piston is still slightly larger at the skirt.

    The new rings are still in the Caber package but I don't know the source of the new piston. The saw does have both Hi and Lo screws.

    Everything else looks like new and I doubt this saw has been run more then a few hours. I'm just trying to come up with theories why the piston has so much skirt wear with so few visual signs. Wouldn't straight gas or an overly lean condition show more signs of damage? All I can do is put it together and continue to find what caused the problem.
     
  12. Mad Professor

    Mad Professor Addicted to ArboristSite

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    Any casting marks on the new piston? Maybe with some pictures someone here can I.D. it.

    Does the cylinder look like it's been worked on? Maybe saw was straight gassed and owner put in a used OEM piston in. Check the stampings on the cylinder and piston , there should be an A or a B on tops.
     
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  13. cuinrearview

    cuinrearview Addicted to ArboristSite

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    It was pointed out that some sanding has already been done to this piston. You can create "wear" pretty quickly on raw aluminum.
     
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  14. dmb2613

    dmb2613 ArboristSite Operative

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    I would like to see inside the cylinder, that piston is worn out.
     
  15. Mad Professor

    Mad Professor Addicted to ArboristSite

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    A good point. Regardless, 0.014" clearance will be going slap slap slap until the skirt separates
     
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  16. cuinrearview

    cuinrearview Addicted to ArboristSite

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    Absolutely. I was just stating that in response to his concern that the "carpet didn't match the drapes" regarding condition. That piston isn't an option.
     
  17. davefr

    davefr Addicted to ArboristSite

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    Here's the cylinder and original piston marking. I think it's possible that the piston could have been sanded by the original owner before he decided to buy a new piston and rings. Those marks on the cylinder's exhaust side don't appear to be scratches. (there's no depth or difference in feel with them).

    In the meantime I took apart the carb and it looks brand new. All passage ways were clear and the needle looks perfect. Both adjustments were at their correct initial position. The impulse line and fuel line look perfect. The air filter shows no signs of fine dust.

    I'm thinking it might have been straight gas'd but the cylinder seemed to survive just fine.

    I guess it could also be a bottom end leak but I'd think that would be unlikley on a like new saw.
    P8120012.jpg P8120014.jpg P8120018.jpg P8120019.jpg P8120022.jpg
     
  18. cuinrearview

    cuinrearview Addicted to ArboristSite

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    That cylinder has been worked on too. It still needs some more cleaning before it is run. It has a little more run time on it than youthink.
     
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  19. davefr

    davefr Addicted to ArboristSite

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    What would you do to clean it up?
     
  20. r black

    r black ArboristSite Guru

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    emery cloth or ball hone the cylinder lightly... install new piston oem or meteor piston perform leak down test inspect lines and hoses... repair as needed ...and cut some wood.... this would be running already if it was mine ;)...
     

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