Misaligned bar on MS390

[WolfpactVI]

Hello everyone,

Sorry I havent posted in a while. What I decided to do was swing by my local Stihl dealer and pick up the Stihl brand rim sprocket and drum kit. Wouldnt you know, it aligned perfectly. Out of curiosity, I took some measurements. Turns out that the Oregon drum, measuring from where it bottoms out on the clutch to where the rim would bottom out against the drum, was a good 3/32" thicker than the Stihl one. I was really amazed by that. While I would generally consider thicker to be better and stronger, if it doesnt fit, what's the point? Now, apparently there are two different clutches that could go on my saw (according to what I read in the service manual), and I dont have the different one, so maybe that fits with the Oregon drum. Who knows. All I do know is the genuine Stihl parts fit well. Now I've actually got to find a spare second to take the thing out and run it again.

As an aside, while I had the saw on the bench I did two things. First, I did a muffler mod. Which is a second reason I'm anxious to get it back outside and run it again. Second, after I did the MM, I use a compression tester to check the saw (the kind you screw into the spark plug hole). I only did it afterwards because that's when I thought I might as well check the compression and went and bought the kit. What I got was ~ 130psi of compression. Seems a bit low to me. Any idea what's normal/acceptable for the 390's? Does doing the MM have any bearing on what the max compression (I dont think it does, but I'm asking anyway).

 

[RedOctobyr]

I'm having the same room sprocket fit issue on my MS290, using a Forester brand setup. I am also planning to buy a Stihl replacement.

MM should not impact compression.

Did you have the choke off, and the throttle fully open, for the compression test?

Does your tester have the air valve right where it screws into the spark plug hole? Any extra air volume before the valve (like if using a thread-size adapter) will make your reading lower.

My used MS290 levels off at 150-155. If the larger bore is the only difference on a 390 (but same compression ratio), I'd expect the results could be compared.

Sent using Tapatalk.

 

[huskihl]

Am ms390, like most, should have 150-170 psi of compression. But 130 is runnable. At about 100 it won't fire anymore. Rings are probably worn out. $15. Fix it next year (or 5) when you're into the saw again. No big deal

 

[WolfpactVI]

Yes, my tester has a 15in hose with a quick attach coupler (like on a compressor) and said coupler has a little bleeder button on it (dial retains max pressure reading until button is pressed). However, the saw is ~10 years old. Wouldnt be surprised if the rings are worn. That'd be a new experience for me, replacing them.

 

[WolfpactVI]

RedOctobyr,

Sorry I didnt take pictures of the two drums to show the dimension differences. I've been squeezing working on it between about 12:15-12:45 am each night, and I was in a hurry to get it reassembled. You'll see on the Stihl drum there's a section machined out on the inside of the drum so it sits deeper against the clutch.

 

[RedOctobyr]

Yes, my tester has a 15in hose with a quick attach coupler (like on a compressor) and said coupler has a little bleeder button on it (dial retains max pressure reading until button is pressed). However, the saw is ~10 years old. Wouldnt be surprised if the rings are worn. That'd be a new experience for me, replacing them.

There should be a little valve in there somewhere, mine looks like the tip of a car tire valve (a Schraeder valve). On mine, this checkvalve is right where it screws into the plug hole, you can see it. The length/diameter of tubing after the valve (between the valve and the gauge) doesn't really matter, that only influences how many pulls are needed to stabilize the reading.

But if you have 15" of tubing *before* this little checkvalve, then your readings will be artificially low, since that length of tubing vents every time the piston goes down, and you can't keep pumping air into it. So it essentially reduces your compression ratio, acting like dead air space that reduces the pressure reading. Unlike the tubing *after* the checkvalve.

RedOctobyr,

Sorry I didnt take pictures of the two drums to show the dimension differences. I've been squeezing working on it between about 12:15-12:45 am each night, and I was in a hurry to get it reassembled. You'll see on the Stihl drum there's a section machined out on the inside of the drum so it sits deeper against the clutch.

Thank you. Just to help link two recent discussions on the same topic, this is my thread about aftermarket rim-sprocket misalignment on my MS290 (effectively the same as your MS390, in this aspect):

http://www.arboristsite.com/communi...-sprocket-wont-align-with-ms290-chain.279019/

In post #23, I included a picture comparing the Forester and Stihl assemblies, showing the issue. It's disappointing that the Oregon setup didn't fit properly; being a bigger name, I was hoping they would get it right.

 

[WolfpactVI]

Thanks for the link. As mentioned in post # 28, it's that pocket on the inside of the drum that's missing from my Oregon drum, thus causing the outside face of the drum (that the rim butts up against) to stick too far outboard of the bar. Again, I must say that it boggles the mind that a manufacturer would make a part claiming it fits a particular saw, yet in reality it doesnt fit, and by quite a bit.

Regarding the check valve, I will **ahem** check it when I get home.