MS460 Piston & Cylinder eater, any ideas?

Cory_T · November 29, 2009, 03:19:47 PM

I have had this ms460 for about four years with absolutely no problems. Love the saw. Late last year when I went to use it and it would not start. Come to find out I had piston failure on the exhaust port side. I asked the dealer and another small engine guy what they thought and they said bad gas. Ok fine. I ordered and installed new piston & cylinder kit, all new seals, and carburetor kit as well since it has never been done. Installed that in July and ran the saw just long enough to cut one tree with no problems. Got it out again in October to start cutting. Ran thru half a tank and herd a click upon letting off the throttle. Would turn over but didn't try to start as I wanted to find out the problem first. Same problem but this time the ring caught the exhaust port and snapped, (that's what the click I heard was). Destroyed the piston and cylinder. The dealer I got the parts from warranted the parts. I installed the second kit and ran ¾ tank of fuel and same thing. I am running synthetic oil and pure gasoline (no ethanol). I just talked to a small engine mechanic, not a Stihl dealer, and he told me that the decompression valve may be bad. He thought that he remembers seeing a recall form Stihl about it but is not sure. I am going to try to contact them this week. Any ideas or anybody heard about the decompression button problem? Would that cause this?

I posted pictures of the piston and cylinder in my gallery.

Thanks -Cory

Al_Smith · November 29, 2009, 04:15:52 PM

Generally speaking a cooked piston indicates one of several things . Either running lean, an air leak or a plugged exhaust in some form or another .

A snagged ring indicates one of two things .Either you got two piston /cylinders that were defective or you installed the piston 180 degrees out . The arrow on the piston goes towards the front .

Cory_T · November 29, 2009, 05:13:16 PM

The ehaxust is not plugged at all. The only thing blocking the exhaust is the screen for a spark arrestor and that is clean. As for the arrow, pointed toward the exhaust port.

I downloaded information on how to be sure if the carb is tuned correctly, (as far as rich/lean) from a different post I found on this site but I have not checked it yet.

I need to get the new parts but I want to get more information about what might be causing the problem. I would like to know if a bad decompression button could be the problem.

Thanks. -Cory

Rocky_J · November 29, 2009, 05:47:29 PM

Most of them leak a little bit, the more oily buildup around them then the more it's leaking, usually. If it's bad then it would allow pressure to escape. Since the cylinder is pressurized then it wouldn't allow air into the cylinder even if it were held open. If your saw is lean seizing then the air leak is from somewhere prior to the combustion cylinder.

joe_indi · December 01, 2009, 05:49:30 AM

Cory,

Forgive me for mentioning this, but that piston does not look like one of Stihl manufacture.
If my eyes are not playing tricks, there seems to be a bit of a tapering machined out at the bottom of the cylinder barrel, and also some machining at the crown, in the squish area, not something found on an original Stihl 046 cylinder.Is it an after market piece?
That cylinder has marks of some serious impacts.
Did you use a new needle cage for the piston or the old one itself?
I have seen similar pistons caused by worn needle cages.
Also check the wear at the big end.
If there is too much of wear, the piston will keep hitting the cylinder on every stroke.
You would hear a metallic 'rrrrrr' when you rev the saw.
Before you install the next cylinder and piston, fit the piston without the rings, bolt the cylinder in place without the cylinder gasket.
Pull the starter a couple of times.There should be no clicking sounds caused by the piston hitting the cylinder top or any other obstructions, it should be just a smooth asthmatic sound.
If you get the clicks or such instead, check out the cause before you proceed any further.
Joe

Cory_T · December 01, 2009, 06:25:54 AM

You are correct Joe the parts are aftermarket from a reliable dealer. Similar to Bailey's. The marks that you see at the top of the cylinder are from factory. I thought the same thing the first time that I opened the package. Both cylinders that I received looked like that. If you mean needle bearing when you say needle cage, yes I did use the same one. They seemed to be in good shape and when I installed the piston pin it seemed to have minimal play that I could tell with the hand. (I know that is not an accurate way to measure play.) I never tore the lower end apart of the saw. I flushed it before rebuilding but that is all. The top of the piston shows absolutely no signs of damage. All three pistons looked like the one I have shown. I really need to hammer down this problem. I paid shy of $800 for the saw and am soon going to be to almost ¾ of that with another build, especially if I do a complete tear down. The question is, is it worth it?

Thanks -Cory

joe_indi · December 01, 2009, 09:13:07 AM

Cory,

Could you post the pictures of all the cylinders that you have.
Try views of the exhaust port from the inside and outside.

Two possibilities come to mind.
#1. Too low a groove clearance for the rings, which would cause the rings to move too slowly in the grooves.If the rings stick out too far when they encounter the exhaust, .....

#2. Couple that with a wider than usual exhaust port and the risks of the rings sticking out multiplies.

My experience with the aftermarket pistons is that it takes a bit more time for the bottom of the rings to seat.
Sometimes this is noticeable by a slight hint of oil wetting at the outlet of the muffler.
Till then carbon travel between the rings down into the crankcase. This carbon could make the rings stick in the grooves at times.
A possible,but messy remedy is to flush out the engine with fuel once in a while.
You need to remove the spark plug, move the piston to TDC, open the throttle wide and pour an ounce of fuel down the carb.
Pull the starter slowly a couple of times. Switch should be in Off position always.
Turn the saw upside down, full open throttle, a couple of pulls on the starter.
The fuel that is thrown out will be full of carbon.
Repeat this a couple of times till the carbon clears out .(now you know why its messy).
Eventually, after some days of running the oily exhaust outlet will turn dry. This indicates that the rings are seating.
But be careful always about going to prolonged full revs without load, the aftermarket will not behave like the original in such circumstances.

Joe

beenthere · December 01, 2009, 01:43:25 PM

Took awhile to find Cory_T pics, so will bring them here for others to view.

Tom · December 01, 2009, 03:53:58 PM

It's been a lots of years since I messed with rings on my old two stroke motorcycle (1964 or so) but I've seen damage like that when there wasn't enough of a gap in the rings. They would heat up, the gap would close, the ring would quit moving and usually break. I think it is much like Joe indicated in his #1 opinion but it puts the fault with the ring instead of the piston.

Cory_T · December 01, 2009, 08:31:13 PM

Joe as far as I noticed, and I can't honestly say that I was looking for it, there was absolutely no evidence of any moisture of any kind near the exhaust. As for the ring gap that you and Tom mentioned it did cross my mind. Thats what I figured happened the second time when the ring actually snapped. I did not check the ring gap, either time. I honestly do not have much experience with this problem that I am having. Can ring gap really cause the piston to look like this or does it seem to be a lean issue?

Believe it or not I emailed Stihl direct and they forwarded the message to Northwest Stihl, located in northern Wisconsin, and they got back to me last night. I called them today but the contact in the service department that I called me was teaching a class today so I told him in his voicemail that I can wait until tomorrow. We will have to see what he says. I will let you know but I am still open for opinions.

Good thing I have the little Husky yet but it's like cutting your t-bone with a butter knife instead of a steak knife.

Thank you beenthere for posting the pictures. -Cory

Cory_T · December 01, 2009, 09:05:43 PM

Here is a picture of the exhaust port from the outside. This is not the factory part, this is the aftermarket cylinder.

Rocky_J · December 01, 2009, 09:19:36 PM

Being an aftermarket jug and piston, Stihl really has no control over it. I suspect if you were to use a $tihl jug and piston then you probably wouldn't be breaking rings.

Al_Smith · December 02, 2009, 06:29:51 AM

Well first of all you are dealing with a set of steel piston rings that are of the pinned type so they don't rotate .It's highly unlikely they were oversized and swelled with heat but I suppose anything is possible .

What is likely by looking at the picture is the rings snagged something .Possibley tramp aluminum that was left over from destroying two pistons prior . It could be inferior plating that broke off the bottom of the cylinder below the ex port . I doubt that Stihl will do anything about it .First because it's not Stihl parts and secondly because the repairs were not made by a Stihl dealer .

Were it mine,which it's not I'd peel that thing down to the bones and give it a physic .Clean the dickens out of that thing .I mean split the cases and check it all out . You could have a bearing full of swarf or any number of things that will reach out and bite in the buttocks when you least expect it to .

Cory_T · December 02, 2009, 06:39:33 AM

I am not trying to get Stihl to do anything about the parts. Even when the saw had 100% Stihl products I had this problem. That is why I am in the situation that I am in. Since they are willing to talk to me I would just like to ask them their opinion. So far I am not comfortable with any one item being the problem. If I choose to rebuild the saw, I want to be confident that I am fixing the saw not just making paper weights out of parts. I don't think it is necessary to list my credentials but I am more than comfortable doing the fix just not with this problem. I have never run across this problem and can't seem to get a solid reason. That's why I joined this site, to help figure it out.

-Cory

Cory_T · December 02, 2009, 06:50:45 AM

Al, that is kind of what I was thinking. I complement you on the knowledge that you have in just being able to look at a part and know the information that you have shared. I will tell you that the cylinder has never been used twice. They were sold as a kit, installed as a kit. As for the rings that is what I had in mind. Looks like it is going to be a complete tear down. I will have to put some cost together and see what it looks like. I flushed the case when I had the cylinder off but I know that is not as good as separating the case.

Let me just repeat, I am not looking for Stihl to do anything about the parts, just looking for their opinion. That's what they are there for.

-Cory

Mad Professor · December 02, 2009, 06:52:16 PM

A retailer had a bunch of Chi-Com P/C assys that had a groove at the top of the cylinder, a deep groove where the nikasil/chrome stopped. The groove was above the rings at TDC but not for the piston crown.

OEM and quality aftermarket (i.e. Italian Tecomec), the chrome/nikasil goes right to the top as does the cylinder itself. In most cases you see the plating on a bit of the dome of the cyl.

You get what you pay for.........

Stihl need to find out why the OEM crapped out.......

Cory_T · December 02, 2009, 07:29:22 PM

You are correct Mad Professor. You do get what you pay for. I purchased it because in the past with other engines that I have rebuilt I did not have a problem. I learned my lesson here. Like you said, I need to find out why the Stihl parts went bad in the first place.

Al_Smith · December 02, 2009, 07:54:28 PM

I don't know what the other two cylinders failed for but that one snagged something on the down stroke .You can tell the way the chip is out of the bottom below the ex port and the ring bent upwards .

In rethinking this thing is it possible you could have broken a little piece off the muffler gasket or something ? Usually if they ingest something it either snags on the intake port or spits it up to get caught in the transfers . Whatever it was it remains a great unsolved mystery .

If it blew something out of the crankcase and made it though the transfers it would blow it out the ex .I suppose it's possible to rattle around in the muffler and get bounced back in .Stranger things have happened .--That mouthfull said I just don't know .

Cory_T · December 03, 2009, 06:54:20 AM

Al you should have seen the cylinder that I had to send back. It was destroyed! Ripped a 3/16" grove in the cylinder right by the exhaust port. I actually found ring parts on the top of the piston when I tore it apart.

Anyway, here are a couple pictures of the gaskets that you mentioned. It was new with the first rebuild. It seems kind of odd to me that the heat shield goes on the cylinder side but that is the only way that it can fit with the gasket.

-Cory

Al_Smith · December 03, 2009, 09:22:28 PM

Part of the right hand bottom portion of the gasket is either cooked or just plain missing according to that photo .

joe_indi · December 04, 2009, 06:29:04 AM

That heat shield and gasket look like they were not in proper alignment.
That is what the carbon on them point to.
Cory that heat shield, it goes onto the cylinder, then the gasket and lastly the muffler.
But, when you fitted that heat shield, did you fit it with the bent portion facing forwards, on the bumper spike side?
Or did you fit it with the bent portion facing backwards, on the starter side.
Please forgive me for asking this seemingly dumb question.
Its because that heat shield (cooling plate) seems to be bent out of shape and it happens usually when you fit it the wrong way.

Joe

Cory_T · December 04, 2009, 07:02:50 AM

Al, the gasket is not missing but it does have evidence of passing exhaust.

Joe, the shield or cooling plate, if that is what it really is, can only fit on the cylinder one way. The pressed pins if that is what I should call them have to go around the cylinder port. The gasket will not fit tight to the plate on that side. To answer that question, the bent part of the plate faces backwards or towards the starter.

ladylake · December 04, 2009, 07:06:36 AM

Not reading this whole thread, I had a snowmobile engine that had the bearings going out, it threw some small pieces on top of the piston, the 2nd time I figured out what was going on. Steve

Al_Smith · December 04, 2009, 04:07:25 PM

Quote from: Cory_T on December 04, 2009, 07:02:50 AM
Al, the gasket is not missing but it does have evidence of passing exhaust.

I know it's not missing as in somebody stole it .Let me rephrase that .

If you look the bottom most picture and in the lower right hand side of the gasket it looks like a tiny little piece is missing .Maybe it's an optical contusion ,illusion confusion ,whatever .

GASoline71 · December 04, 2009, 06:36:09 PM

Also... just an observation... if it was originally diagnosed with as "bad gas" being the problem. I'm just curious if you continued to run the rebuilt saw on the same batch of fuel.

I have done a lot of work on friends saws that "ran like crap", or were actually damaged. I would rebuild carbs... relpace pistons, jugs, rings... and then put my fuel in the saw, and they would run like a top. Give them back to their owners who would put their 2 year old gummed up fuel in them and come back 2 weeks later. "My saw runs like crap again."

Just wonderin'?

Gary