Stihl MS440 Magnum Low compression?

[flhx]

Newbie checking in here; I just received a Stihl MS440 Magnum from a friend. It's a non runner. The first thing I checked was spark (good) and compression (90 PSI) I removed the exhaust and the piston does have a little scoring, the rings appear to have a little scoring too. I'm thinking of just replacing the piston & rings.

My understanding is anything under 110 PSI is unacceptable.

* Any and all technical assistance will be greatly appreciated.

Thank you all in advance

[Texas-Jim]

It can run at 90 maybe but it wont ever be a very good cutter. If the rings are scored chances are the cylinder is as well. The rings are very hard and for them to score they always as a rule damage the cylinder.

[blueberrymuzik]

90 psi isn't good. You might check the compression release valve and make sure that it is pulled up- then try your compression check again. Some pictures of the piston and rings might be helpful, but it sounds as though the cylinder will need to come off and the piston & rings replaced. Unless something catastrophic has happened, you can usually clean the aluminum transfer off the cylinder walls, install and new piston & rings and you are ready to go again. One other very important item should looked into- why is the piston scored? Generally, after years of use, the crankshaft seals need to be replaced. When they leak, they allow air to enter the combustion chamber. This creates a very lean fuel/air mixture and will cause scoring. Other items that can also cause scoring. 

[ZeroJunk]

That's a saw that is worth rebuilding. It is unusual not to be able to clean a cylinder. A little muratic acid and Q-tips. Some 400 or thereabouts grit paper and the end of your finger. If you can't get it completely slick below the ports it is not a deal breaker. And, despite the dogma you read on forums a few seconds in and out with a ball hone won't hurt a thing. Replace seals, impulse line , intake boot as a matter of course. Should get a lot more use out of it.

[flhx]

Thank you for the intel. It's greatly appreciated.  I plan on keeping the saw, so I plan on a purchasing a complete overhaul kit.

Thanks again

[joe_indi]

I would open it up and do a through check. If, as you say, the scouring is slight, chances are you can re-use the current piston and rings.
Careful removal, a bit of buffing with emery and switching the rings should give you the required compression.
But, before you try starting it the main job is trying to find the cause of the original piston siezure and doing the proper repair for it.
Joe

[Canadiana]

My limited experience has been rings show the most wear, piston is next and cylinder shows the least amount of wear. Rings make contact the entire time the engine runs. The cylinder gets an opportunity to cool as it is only being touched as the rings go by. I'm hoping someone experienced will correct me or agree:)