MS 660 Mystery

[Forallhesgot]

Hi All,

I'll admit I'm not an expert - but I'm a bit stumped here, that's why I am asking the experts!

I bought an MS660 recently - the saw is in pretty good shape, ran well, cut well etc. when I tested. When I got home I pulled the muffler to inspect the cylinder and found a little wear on the exhaust side. Nothing too concerning, but I was up for a project - so I decided to pull the cylinder, clean it up and install a new meteor piston + rings.
Now when I first bought the saw it did run well, just felt like it may be a tiny bit low on compression as it was easy to start.
I cleaned up the OEM cylinder with scotchbrite, it cleaned up very well. There was a lot of carbon build up, on the head and exhaust. At the same time, I gutted the muffler and matched the exhaust outlet with the gasket and muffler. I installed a new OEM fuel filter, new Maxflow filter.

After the rebuild, I pressure + Vacuum tested the saw, checked the fuel lines, carb, fuel vent, impulse line. I checked a few times on all parts - it holds pressure and vacuum very well.

After the rebuild I ran it only with Stihl pre-mixed non e gas, to eliminate that variable - I ran almost 2 tanks though it and decided to pull the muffler + plug to see what was going on. It should be said that the saw ran great, the only issue is the idle would vary from a little low to a little higher after cutting, not sure if this is my tuning, or I;jm being overly sensitive but it would idle without stalling and ran well with lots of cutting power etc.

So when I pulled the plug and muffler I did find some marking, although very light it was concerning. Here are the pics after I pulled the cylinder (this is after the original rebuild + new piston)






These marks came out with a little hand sanding and scotch-brite - cleaned up very well with the exception of a tiny vertical scratch, this is mainly on the exhaust side (just like the OEM piston). Cant catch my fingernail on anything.

So I decided to run another tank through it, I pulled the plug to peer into the cylinder and I'm seeing very similar markings on the cylinder walls after 1 tank.

Now I have pressure + vacuum tested all parts of the saw several times, to try and eliminate that. the carb was cleaned and tested as well.
The bearings feel very good, no play and smooth. I also looked at them best I could with the cylinder off and they appear fine.. I cleaned out the case the best I could and also tried my best to have a clean working environment.

Now I'm a little stumped here.. The saw runs very well, I can't find any issues when running it with the exception of the idle being sometimes a little high or a little low after being on full throttle. The saw is tuned rich - its 4stroking a bit and I'm paying very close attention, doesn't seem to be leaning out in the cut or in high rpms. I'm also monitoring via a tach, but listening intently as well.
Any ideas?? I've searched high and low, can't seem to find any clear ideas.. Of course bearings jump out at me as a possible one, but can they be shot with no other obvious signs? Also after I took the saw apart the second time, I didn't see any obvious pieces of metal shavings (would there be a lot, or just a tiny bit?).

Here is looking through the spark plug hole now at the cylinder (this is after approx. 1 more tank, since second cylinder cleanup). The new piston still looks good, can't see hardly anything different on it.
I know its hard to see, but there are lines in there, I don't think they are deep (they cleaned up easily the first time)



I did not pull the clutch or flywheel off to check the bearings + seals, because they felt tight + smooth, when I flushed out the crankcase, I didn't see a lot of gunk or anything like that.

Of course the is my first expensive saw, so I want to get this work right and not ruin my purchase! I'm also a bit bummed that my first rebuild went this way, but sometimes that's the way she goes.. I would continue running and hope for the best, but I don't want to ruin anymore good OEM parts! Also this saw will pretty much be for milling, so I want to make sure its tip-top before I put it to use. For now I have just been running it for regular cutting to break in before it will go on the mill

Thanks in advance for your help, sorry for the novel haha!

 

[Gary Courtney]

are you oiling the rings and cylinder as you put together? Also make sure your hands are not filthy whe handling piston as grime you may not see can get on the piston skirts,rings and cause this. otherwise just run it.

 

[Forallhesgot]

Hi Gary,

Yes I put a good smattering around the rings, piton and cylinder walls upon re-assembly. Also I took a lot of care to keep dirt or grime out of the area, and off my hands.. but could have been a little something sneaking in.
Thanks!

 

[Deets066]

What’s your ring end gap?

 

[Nutball]

If you can't feel anything, could it be the normal breaking in of new rings just polishing the cylinder?

 

[Forallhesgot]

Ring end gap I did not check - should I pull it apart to check this ? The rings are brand new caber..

Also I am wondering if this is normal wear - however it doesn't seem normal (although I may not know what normal looks exactly like , seems like something is wearing wrong..

I guess the Million dollar question is if I keep running it, will I ruin the OEM Jug, and/or the bottom end (or something else expensive)? Or will I be okay..

Thanks for the ideas everyone!

 

[Wonkydonkey]

I read this thread before, and looked at your pics,, I was a bit stumped as to why and what.. but now I can’t seem to see you piston pics.

How ever... before I thought idk and now I still don’t know. And can’t relook now The only other thing I can think of is that you did not remove all the transfer the other thing was, I thought your intake was close to your ring end and maybe somthing chipped off..

Hopefully you can sort it..

 

[Nutball]

Just to help put your mind at ease here's a new echo cylinder after 30 seconds to 2 minutes run time. The marks can't be felt.


I do have a cylinder with 5 minutes of run time with scoring that can be felt due to something wrong I did, but if you can't feel it, don't worry. I checked my other 2 saws I modified in case they were bad too simply by sticking a carbide scribe through the spark plug hole and gently running it along the cylinder to see if the lines can be felt. It saves the effort of frequent tear downs if you want to check up on the wear in the future.

 

[67L36Driver]

What’s your ring end gap?

+1

One must account for thermal expansion.

About .005” would be good.

 

[CR888]

When vac/pressure testing did you turn over the crank back & fourth? I'd be running more oil to after a rebuild. A good mineral oil @32:1 and tuned correctly for the oil ratio.

 

[Ketchup]

Sometimes new rings have swarf from the manufacturing process still clinging to them. I always wipe them down with oil and look for shavings before install. Something like that could cause scratches.

Keeping all this in mind, I think you should run it for a while. It was a running saw before you started into it and it's a running saw now. You risk as much repeatedly dismantling it as you do running it and waiting for a problem to surface. Many cylinders show lines and still run fine.

 

[Nutball]

After running a few new saws, and looking into the cylinder of other people's low use saws, I'm starting to think minor scoring is normal more or less. After lots of use , as I've looked into some high use old saws too, they seem to wear down those scratches to the point of looking new, though with a somewhat wide line of oil that slips through the expanded ring gap. If new rings wouldn't risk more scoring, I'd say run a few years, then change the ring.

 

[Forallhesgot]

Thanks everyone for the replies!! sorry on my delay I’ve been traveling for work a bit..I think perhaps I was being overly sensitive, or maybe the rings are scritching the cylinder up a bit, maybe a speck of dirt? Who knows, but certainly the responses are helping for piece of mind!

I’ve decided to just run it, seee what happens and just pay close attention to see if anything noticeable surfaces.

As for the oil mix, I am running 40:1 and trying to keep the tune very rich.. also I did turn the crank over while pressure testing. And tested several times.

The saw seems to be running great, it’s definitely strong - I’ve got a few more tanks through it.

There is one more mystery however - as the idle tune seems to change drastically on the saw from warm vs cold.
For example, when I’m running the saw on the mill (especially when it’s horizontal, but somewhat while I’m cutting normally with the saw vertical) I have to turn in the LA and L screw quite a bit to keep it idling while it’s warm.
Then when I shut off the saw and restart it totally cold, it’ll idle above 3k rpms with the chain spinning FAST. On this same tune it idles super low when hot, throttle response still good, in the low 2k range.

I do have a carb rebuild kit ordered, but saw no obvious issues when cleaning carb, also pressure/vac tested all parts while I did this (everything except jets, I haven’t touched them yet).

Any thoughts?

Thanks in advance!

 

[Adirondackstihl]

Thanks everyone for the replies!! sorry on my delay I’ve been traveling for work a bit..I think perhaps I was being overly sensitive, or maybe the rings are scritching the cylinder up a bit, maybe a speck of dirt? Who knows, but certainly the responses are helping for piece of mind!

I’ve decided to just run it, seee what happens and just pay close attention to see if anything noticeable surfaces.

As for the oil mix, I am running 40:1 and trying to keep the tune very rich.. also I did turn the crank over while pressure testing. And tested several times.

The saw seems to be running great, it’s definitely strong - I’ve got a few more tanks through it.

There is one more mystery however - as the idle tune seems to change drastically on the saw from warm vs cold.
For example, when I’m running the saw on the mill (especially when it’s horizontal, but somewhat while I’m cutting normally with the saw vertical) I have to turn in the LA and L screw quite a bit to keep it idling while it’s warm.
Then when I shut off the saw and restart it totally cold, it’ll idle above 3k rpms with the chain spinning FAST. On this same tune it idles super low when hot, throttle response still good, in the low 2k range.

I do have a carb rebuild kit ordered, but saw no obvious issues when cleaning carb, also pressure/vac tested all parts while I did this (everything except jets, I haven’t touched them yet).

Any thoughts?

Thanks in advance!

Sounds like the L is too lean

 

[HYPERSAWS]

Molecule overload