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Ms390 rebuild questions

Discussion in 'Chainsaw' started by motorhead99999, Oct 10, 2019.

  1. motorhead99999

    motorhead99999 ArboristSite Lurker

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    i am curently rebuilding a ms 390 that had low compression and minor scoring so I ordered a cross pop up with caber rings. Took the saw all apart and cylinder is junk so I ordered a hyway cylinder. But I was wondering if anyone has attempted to seal the pan to the cylinder out of the saw and then take the bolts out of the pan and put back in the saw. It just seams like it would be a little easier to not cover everything in Motoseal. Also should I replace the crank bearings while I’m in there. Thanks
     
  2. Poleman

    Poleman Addicted to ArboristSite

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    I always assemble the pan to the cylinder out of the case then put it in and torque it down.
    Replacing bearings isn’t always necessary but you will need to clean and inspect and feel for any roughness, cracking of the ball retainer or any wobble or looseness feel.
    I always replace crank seals anytime ones apart for maintenance. Cheap insurance.
     
  3. MontanaResident

    MontanaResident Addicted to ArboristSite

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    That is pretty much how I would do mine, with a tiny amount of sealant between pan and cylinder before final assembly.
     
  4. a. palmer jr.

    a. palmer jr. Tree Freak

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    I do mine a little differently. I assemble the engine then put the pan on with Motoseal or Dirko and bolt it down, let it set overnight, take the bolts out and then put the engine in the saw. It's a little different but it works for me and I've never had a failure...
     
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  5. Overkill338

    Overkill338 Bars Of Stihl

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    I've done three 290 to 390 swaps with Hyway kits. I assembled the engine outside of the case and left it 24 hours before bolting it back in.
     
  6. motorhead99999

    motorhead99999 ArboristSite Lurker

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    This is how I was thinking of doing mine and what my question was. Thanks
     
  7. motorhead99999

    motorhead99999 ArboristSite Lurker

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    This is the cylinder off of it looks like it was straight gassed to me.
     

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  8. motorhead99999

    motorhead99999 ArboristSite Lurker

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    And it still had 125 psi compression
     
  9. a. palmer jr.

    a. palmer jr. Tree Freak

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    125 compression is a little low, it's definitely needing some work.
     
  10. motorhead99999

    motorhead99999 ArboristSite Lurker

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    Do you think I can clean the cylinder in the pic up enough to re use it
     
  11. Overkill338

    Overkill338 Bars Of Stihl

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    I have a Hyway 390 cylinder kit.
     
  12. a. palmer jr.

    a. palmer jr. Tree Freak

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    Kinda hard to tell by the picture but if the grooves in it are even a little deep I'd replace it, mainly because aftermarket cylinders are so inexpensive now, you can buy the whole kit for 20 some dollars on the bay. Just be sure to oil everything when putting it together and don't run it too lean while breaking it in. I use about 40:1 with the aftermarket kits, using Stihl oil and good, non ethanol gas..
     
  13. motorhead99999

    motorhead99999 ArboristSite Lurker

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    I run ethanol free in all my small stuff. I run 40:1 normally in my two stroke stuff. Summertime I run castor 927 winter time I run amsoil. I have tons of racing oil laying around so I burn it in my saws
     
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  14. motorhead99999

    motorhead99999 ArboristSite Lurker

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    How much do u want for it. I ordered one last night with piston and rings but never know when the old 290 will give up.
     
  15. motorhead99999

    motorhead99999 ArboristSite Lurker

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    By the way what does stihl use for base sealing. It just looks like normal permatex to me.
     
  16. a. palmer jr.

    a. palmer jr. Tree Freak

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    Stihl uses Dirko. It's red in color and is a little pricey. Permatex Motoseal can be used as well and is a bit cheaper and less messy.
     
  17. Overkill338

    Overkill338 Bars Of Stihl

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    $55 shipped
     
  18. motorhead99999

    motorhead99999 ArboristSite Lurker

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    Is it new or used and how would I pay you im new here so not sure how it works yet
     
  19. Overkill338

    Overkill338 Bars Of Stihl

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    I sent you a PM bro.
     
  20. PogoInTheWoods

    PogoInTheWoods Don't forget about the alligators...

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    I just happen to have an example handy...

    1127 Shortblock.jpg
    I use Dirko. You can put one of these together with Dirko, bolt it in and run the saw right away with no problem. I still bolt it together outside the engine housing for the main reason of doing a leak check before committing to complete reassembly..., especially aftermarket kits or cylinders. The machining and casting is all over the map with the cheapy kits and cylinders and more will leak than not unless excessive sealant is used. Some say to always use the old OEM bottom pan but I've found using the pan that comes with the cylinder can often be a better match for the seal pockets. Regardless, it's nice to know you have a completely tight shortblock beforehand than finding out the hard way there's a leaky seal once the saw is back together AND you have that damned bar stud screwed back in! BTW, I just use Gorilla tape to block off the intake and exhaust ports for a quick vac/pressure test outside of the engine housing. Works fine up to around 5~7 psi and 12~15 inHg which is all that's necessary anyway. Intake and exhaust surfaces just need to be nice and clean for a good seal. I use a little alcohol. Another good reason for sealing outside the housing and letting it completely set up is for ease of getting the assembly back into the housing itself. It's just easier when you're not trying to keep the pan from sliding around while also trying to position the assembly.
     
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