MS660 scored piston advice

[jeepinmatt]

I recently acquired a non-running MS660 from a friend. I was there when it died. He was finishing the back cut on a 50" oak trunk that we were dropping, and it just stopped mid cut and wouldn't recrank. I finished the cut with my trusty old 046.

I know the gas he was running was properly mixed 50:1 for a multitude of reasons, the biggest being that he was running his other 2 saws out of the same fuel can with zero issues. So I checked the obvious things. It's getting spark, has fuel, and seems to be getting it to the cylinder. Carb is perfect (he had recently acquired the saw and replaced a few things including the carb). Air filter is clean. BUT, compression is 110psi, and the exhaust side of the piston is very scored.

So, the price was right and now it's mine, and now it needs new life breathed into it. Which leads me to several questions:
1. Would 110psi comp and a scored exhaust side cause it to not run? 1 out of 25 pulls you hear it almost hit and run, but it never actually does.
2. Assuming it needs a new piston and cylinder, what's the hot ticket? NOT concerned with doing it as cheap as possible, but I don't want to waste money either.
3. I will probably want to get it ported. I have a Woodmizer LT30, and want something to trim oversize logs down on a couple sides to fit the mill, plus I occasionally cut down 30"+ trees and want something that wears a 36"ish bar and is stronger than the 046Mag for those situations. What's the ballpark cost for a decent port job? $100 or $500 or something in between?
4. If I'm sending it off to be ported, would it make sense for the guy doing the port work to also supply the top end and basically go through the saw?

Thanks in advance!

And here's a few pics of the 660 killer since everybody likes pics

 

[Vintage Engine Repairs]

It’s likly an air leak pressure and vac test.
What are the settings on the carb L and H screws?

 

[jp233]

Sounds like the piston is toast. The cylinder might be salvageable with some muriatic acid. 110psi is not that great. For a great big saw like that.... make it scream. You might be able to send it out and have them fit a new piston, do the porting and you'll have a real ripper. Obviously doing crank seals and stuff might be a good idea - just have them go through the whole saw if you can send the whole saw out. But you might be able to just send the cylinder and have them port + do the top end, you do the rest?

 

[Yukon Stihl]

You can clean up the cylinder without acid.
Using acid is OK if you are sure of what you are doing.If you haven't used acid before you can destroy a cylinder by using acid if it has any defects.The acid will get under the plating and lift it off.
Using sandpaper and a drill or dremel is safer and works just as good.
There are many saw porters/mechanics that will never touch a cylinder with acid.

 

[Vintage Engine Repairs]

I have cleaned and saved 4 cylinders so far just using 240 and 400 grit sandpaper. It works well

 

[Bob Hedgecutter]

Take the plug out- pour a teaspoon full of mix oil down the plug hole, swill it around, put the plug back in and try to start the saw.
If it fires up- it will smoke like a race truck- but will let you know it is just compression stopping you going.
Then you just have to find out what cause it to score up the piston- correct that, hopefully clean up the cylinder, at the very least re-ring it, safer re-piston and ring it, get it running and see if it will do what you need, then consider porting it or having the work done.
A stock 660 is a strong saw compared to the 046 you have, could be the step up is enough without the additional cost of porting work done by others.

 

[jeepinmatt]

As far as I can tell, a piston is $50, and whole new top end is about $100. It's worth $50 to me to not have to take it apart again and risk ruining a brand new piston. In my younger days when I was a cheap bastige with time to burn, I would pull it apart and clean everything up and try to limp it along. Nowadays, my time is much more valuable and I'd rather do it once and do it right so that I don't have to deal with it again. I'm quite mechanically inclined, but I'm completely unfamiliar with how to

pressure and vac test.

a chainsaw. I'm sure I can search on here and figure it out, but if I'm gonna send it off to be ported, am I just wasting my time?

 

[jeepinmatt]

Take the plug out- pour a teaspoon full of mix oil down the plug hole, swill it around, put the plug back in and try to start the saw.
If it fires up- it will smoke like a race truck- but will let you know it is just compression stopping you going.

I did the "half-ass" version of this yesterday. Shot some WD40 in there. Didn't seem to help. I'll try it again with the heavier pre-mix oil.

I guess the main crux of my question is whether or not low compression would cause it to not run. I would certainly think so if it were like 30psi or something really low, but 110psi isn't that far off from a good running saw. I would expect reduced power of course, but not to die in a cut and never recrank. So I want to make sure I'm barking up the right tree.

 

[Bob Hedgecutter]

I did the "half-ass" version of this yesterday. Shot some WD40 in there. Didn't seem to help. I'll try it again with the heavier pre-mix oil.

I guess the main crux of my question is whether or not low compression would cause it to not run. I would certainly think so if it were like 30psi or something really low, but 110psi isn't that far off from a good running saw. I would expect reduced power of course, but not to die in a cut and never recrank. So I want to make sure I'm barking up the right tree.

All depends on how true the tester you reached your 110 is, but yes, that low will stop a saw from running- generally speaking, less than 120 psi and less is going to be in the no run area.
WD40 is too thin to perform the test.
That is exactly why I suggested doing the test- to see if you are barking up the right tree or if there are other problems. Many a compression tester will give inaccurate readings on a small 2 stroke motor, using oil to take up the compression slack allows you to somewhat prove the reading is correct.
Of course the other option is, use the same tester on your 046 and see what the reading is.

 

[Yukon Stihl]

As far as I can tell, a piston is $50, and whole new top end is about $100. It's worth $50 to me to not have to take it apart again and risk ruining a brand new piston. In my younger days when I was a cheap bastige with time to burn, I would pull it apart and clean everything up and try to limp it along. Nowadays, my time is much more valuable and I'd rather do it once and do it right so that I don't have to deal with it again. I'm quite mechanically inclined, but I'm completely unfamiliar with how to a chainsaw. I'm sure I can search on here and figure it out, but if I'm gonna send it off to be ported, am I just wasting my time?

The prices you mention are not for OEM parts.If you decide to go that way don't expect the saw to run as good as it could or last very long.
If you plan on sending it away to be ported then if you are not mechanicly inclined,just send it the way it is.Then the saw porter can do the diagnoses,and get the parts that they are comfortable working with.
Not many porters that do work for other people will be comfortable using any aftermarket parts.They may have a few they are comfortable with,chances are they are not a $100 top end.So you will be throwing away money on that junk.

 

[jeepinmatt]

All depends on how true the tester you reached your 110 is, but yes, that low will stop a saw from running- generally speaking, less than 120 psi and less is going to be in the no run area.
WD40 is too thin to perform the test.
That is exactly why I suggested doing the test- to see if you are barking up the right tree or if there are other problems. Many a compression tester will give inaccurate readings on a small 2 stroke motor, using oil to take up the compression slack allows you to somewhat prove the reading is correct.
Of course the other option is, use the same tester on your 046 and see what the reading is.

046 is in the 160psi range. Friend I got the saw from also tested the 660 at 110psi with his gauge. My background is engineering, machining, and metrology, so to see two different brands of testers within a couple psi makes me feel pretty good about it.

 

[jeepinmatt]

The prices you mention are not for OEM parts.If you decide to go that way don't expect the saw to run as good as it could or last very long.
If you plan on sending it away to be ported then if you are not mechanicly inclined,just send it the way it is.Then the saw porter can do the diagnoses,and get the parts that they are comfortable working with.
Not many porters that do work for other people will be comfortable using any aftermarket parts.They may have a few they are comfortable with,chances are they are not a $100 top end.So you will be throwing away money on that junk.

Correct, which is why I asked:

2. Assuming it needs a new piston and cylinder, what's the hot ticket? NOT concerned with doing it as cheap as possible, but I don't want to waste money either.

Is OEM the only way to go for that stuff, or is there a high quality aftermarket version that doesn't have the Stihl markup?

 

[Yukon Stihl]

OEM is the best for cylinders
Meteor pistons are second choice after OEM in general.There are a few other AM pistons that shine but again that would be for your porter to decide what he wants to put his reputation behind,some are pretty fussy because they stand behind their work 100%.
Others don't care ,and you may never see your saw again,so it's your choice on the quality of work you want done.
If porting is the end result find your porter now and begin the discussion on the saw.Chances are they will want it as is so they can determine the cause of the failure.

 

[cuinrearview]

If it were mine I'd attempt to save the original cylinder. If I couldn't, I'd put a want ad in our trading post for a good used OEM jug and pick up a Meteor piston for it. You should be less than $50 more than the aftermarket top.

 

[Mad Professor]

Post a picture of the cylinder /piston inside with muffler off.

You get what you pay for with P/C sets. OEM is best. A good used OEM cylinder is better than "Chinesium".

Acid method works fine for removing transfer. You just can't have any holes in the plating or get it on to a port chamfer. Probably not a cylinder you'd invest a port job in anyway.

It oxidies aluminum, the hydrogen in the acid gets reduced and makes H2 gas, that's the bubbles. Use a long q-tip to apply and have baking soda solution handy if you splash some acid, and when you need neutralize it. Also, it won't take off grease/oil or carbon. Clean with wet/dry and dish detergent to remove that first.

 

[jeepinmatt]

OEM is the best for cylinders
Meteor pistons are second choice after OEM in general.There are a few other AM pistons that shine but again that would be for your porter to decide what he wants to put his reputation behind,some are pretty fussy because they stand behind their work 100%.
Others don't care ,and you may never see your saw again,so it's your choice on the quality of work you want done.
If porting is the end result find your porter now and begin the discussion on the saw.Chances are they will want it as is so they can determine the cause of the failure.

Looks like a Meteor piston kit is $50 and piston/cylinder kit is $130. https://www.hlsproparts.com/searchresults.asp?Search=ms660+meteor

But it sounds like the primary reason to save the cylinder is that the Stihl stuff is the best option, which I'm good with that logic.

 

[jp233]

Yeah, you're like me.... I had much more time on my hands earlier in life and I'd do almost everything myself. But now I'm too busy with work and kids and stuff. So I dropped off my 028 AV Super at a shop the other day. It needed a bunch of little things, and I just wanted to get it sorted out - for guys who work on Stihls every day, and I've had to only wrench on it twice in the past decade.... might as well have it done once, done right.

You could send pics of the piston scoring to some potential porters, and get their thoughts too, and recommendations on parts. I've heard good thing about Meteor pistons too. Doesn't Mahle make pistons for Stihl saws too? The porter might want to make exhaust mods to match his cylinder porting too, so it's maybe just the best thing to send it out as a whole unit.

 

[jp233]

Acid method works fine for removing transfer. You just can't have any holes in the plating or get it on to a port chamfer. Probably not a cylinder you'd invest a port job in anyway.

Yeah, that was my thought based on the original post or two. If the scoring isn't horrible, and just some piston material transfered to the cylinder wall, etch the bad stuff with a tiny bit of acid, then sand it or quick hone somehow and be able to just replace the piston, do some crank seals because why not, and have a Coke.

If the ring hooked a port, it may need to be opened up anyways, but also if the plating is damaged it's a goner I would assume. I assume chainsaw cylinders aren't worth sent out for re-plating like bike cylinders (Millenium)

 

[Mad Professor]

Yeah, you're like me.... I had much more time on my hands earlier in life and I'd do almost everything myself. But now I'm too busy with work and kids and stuff. So I dropped off my 028 AV Super at a shop the other day. It needed a bunch of little things, and I just wanted to get it sorted out - for guys who work on Stihls every day, and I've had to only wrench on it twice in the past decade.... might as well have it done once, done right.

You could send pics of the piston scoring to some potential porters, and get their thoughts too, and recommendations on parts. I've heard good thing about Meteor pistons too. Doesn't Mahle make pistons for Stihl saws too? The porter might want to make exhaust mods to match his cylinder porting too, so it's maybe just the best thing to send it out as a whole unit.

Mahle and Kolbensmitch (sp?) were suppliers for stihl pistons. Not sure on new ones, my saws are >20 years old.

Tecomec when made in Italy were OEM quality, they went Chi-Com. A recent post indicated Meteor cylinders were now Chi-Com too? I thinks pistons are still Italian? This would be good to know if anybody has a true answer.

You won't go wrong with OEM. Make sure the rest of the saw is R + R ed. Rubber parts, seals, carb.

 

[tug]

I guess the main crux of my question is whether or not low compression would cause it to not run. I would certainly think so if it were like 30psi or something really low, but 110psi isn't that far off from a good running saw. I would expect reduced power of course, but not to die in a cut and never recrank. So I want to make sure I'm barking up the right tree.

Yes, if the ring is stuck they generally don't run. If you pull the muffler and see scoring you likely have something that needs addressed. If you have very minor scoring or some carbon scoring they will run. I would guess trying to repair the cylinder by sanding. Meteor or OEM. I think an OEM kit is $300 and close to $200 to have a stihl shop put it together. You will need to find out if it has an air leak.