MS261 Comprerssion Quandry

[John Lyngdal]

Picked up a MS261 about a week ago. During the pre-purchase checkup I measured the compression at 135 psi, so I pulled the muffler to look at the piston skirt and cylinder bore. The piston skirt looked ok and the cylinder looked to have a a little blush coloration to it. The saw was filthy, so I cleaned it up and tore it down so I could pull the cylinder. Note, the 261 tear down is a PITA compared to a 026. The inside of the jug looks unblemished and the piston is free of scoring. At this point, I wish I had pulled the decompression unit and replaced it with a plug to see if it was leaking and causing the low compression measurements. This is the first strato saw I've disassembled, so I'm not sure if this discoloration on the piston is normal or the result of ring blow by.

The ring protrusion seems smaller than normal, but it might just be my mind plating tricks with me.
What do you think?

If I decide to install new rings, should I use OEM or Caber?

 

[vortec325]

Those things are deff a pain to tear down vs the older saws !


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[Conquistador3]

I seem to recall some conversations from a couple of years ago about similarly unexplicable low compression on a few MS241 and 261 which were otherwise running pretty well. Values between 130 and 140psi were reported, exactly what you are seeing, but the saws were found to be in perfect working order when torn down and pressure tested.
To the best of my knowledge this "issue" was never truly solved and appears not to affect the saws themselves but perhaps there was some later discovery I missed.

Your saw looks in excellent shape to me, and little used to boot. Personally I would just try running it as is for a while and see if it feels down on power or has any other issues. These are the true workhorse saws around, every landscaping service seems to be running them and the local Stihl dealer shift them by the dozen. I may try ask them next time I go there.

 

[drf255]

John, if the saw runs well, I wouldn’t worry about a number.

Are you sure your tester is working correctly? Does it have the white Schrader valve in its tip?

I used to get hung up on compression numbers. They are just a relative thing, if the saw runs well and isn’t scored, it really doesn’t matter.

If you squirt a little oil in the cylinder, does the compression come up?

 

[blsnelling]

I seem to recall some conversations from a couple of years ago about similarly unexplicable low compression on a few MS241 and 261 which were otherwise running pretty well. Values between 130 and 140psi were reported, exactly what you are seeing, but the saws were found to be in perfect working order when torn down and pressure tested.
To the best of my knowledge this "issue" was never truly solved and appears not to affect the saws themselves but perhaps there was some later discovery I missed.

Actually, the MS261 usually has higher than average compression, up around 170PSI.

 

[Vintage Engine Repairs]

My ms 260 with new piston but cleaned cylinder is at 180 psi. You could always slap a meteor piston and caber rings on and see what you get? From what people have said to me (pro loggers and guys that have done this longer than I’ve been alive), the 026/260 (and I don’t see why the 261’s would be any different) are known to have high compression. 170-190 is common.

 

[Vintage Engine Repairs]

.

 

[John Lyngdal]

Bolted the cylinder back on and measured the compression with and without the compression release installed and it might do 140 with a coat of oil on the rings.
Checked the compression on anther saw with known compression and it measured 185, or about the same last time I measured it.
Measured the squish at .040".
Ordered a set of Caber rings for it and we'll see if that helps.

 

[pioneerguy600]

Bolted the cylinder back on and measured the compression with and without the compression release installed and it might do 140 with a coat of oil on the rings.
Checked the compression on anther saw with known compression and it measured 185, or about the same last time I measured it.
Measured the squish at .040".
Ordered a set of Caber rings for it and we'll see if that helps.

That is terrible Squish clearance, does it have a double thick base gasket?

 

[Jkstihl]

That is terrible Squish clearance, does it have a double thick base gasket?

Thats factory squish on a 261 if i recall. But yea id pull the gasket and check it again.

 

[LonestarStihl]

Just checked my 261 for grins and giggles and it was 170#

 

[Vintage Engine Repairs]

Thats factory squish on a 261 if i recall. But yea id pull the gasket and check it again.

If that really is true, that’s shocking.

my squish is .024” and I wanted mine lower. I should just be thankful by the sounds of it hehe.

 

[Jkstihl]

If that really is true, that’s shocking.

my squish is .024” and I wanted mine lower. I should just be thankful by the sounds of it hehe.

My 046 was .30 something stock removed the base gasket and she dropped to .19 kinda close but no issues yet, next mod is to advance timing by shaving the flywheel key.

 

[Conquistador3]

Wait a minute... you people are telling me you test compression on an engine just for fun and not just when something is wrong?

And here I thought I was the one with no life at all.

 

[farmer steve]

Wait a minute... you people are telling me you test compression on an engine just for fun and not just when something is wrong?

And here I thought I was the one with no life at all.

 

[blsnelling]

I don’t see why the 261’s would be any different) are known to have high compression. 170-190 is common.

Because they are entirely different saws.

 

[John Lyngdal]

Because they are entirely different saws.

This would be painfully obvious to anyone who had done a teardown of a 261.

 

[Vintage Engine Repairs]

This would be painfully obvious to anyone who had done a teardown of a 261.

Haha yeah I haven’t done one!

 

[John Lyngdal]

The installed OEM rings measured .066" and the new Caber rings measured .072".
Installed the new caber rings and the compression is in 140-145 range with a little wd-40 in the cylinder.
The cylinder gasket measures .015-.016", so clearly not a double gasket issue causing the lower than expected compression.
I can purchase a Hyway pop-up piston for $30 if I wanted to increase the compression.
Not really sure if this would be worth the effort, but I don't want to tear this saw down again as it a PITA undertaking.