044 broken piston skirt

[lledwod]

Hi there. I just joined this site. I am spending some time this winter getting all my saws in good shape. I dug out my Stihl 044 that broke a skirt on me a few years ago. I had put a new piston in this saw about a year before the skirt broke, but had few hours on it, probably 10 or less. The reason I replaced the piston was scoring due to a cracked intake rubber/boot. The cylinder still seems fine. My question here is, is it possible this is a defective piston? I don't recall in the past seeing a skirt quite so thin, or thinner on one side than the other? If this is just the way these pistons are built, I wonder what else the cause may have been .... I hate to rebuild it and have this happen again!

Thanks for any help.

Jonathan


 

[sablatnic]

Seen this before. Check air filter, intake boot, pulse line, cylinder and crankshaft seals.

[ZeroJunk]

My bet would be defective. What brand was it, or was it OEM ?

[lledwod]

This is an oem,Stihl piston. The intake boot was new when the piston was replaced. I am not sure what trouble my air filter would have caused, I am pretty 'up' on all my maintenance. Like I said, the cylinder seems to be fine, there is no scoring to speak of. I will take a closer look at everything, and maybe see if my Stihl dealer can check out the piston for me.

[deerslayer]

I have seen broken piston skirts in the past and usually it is on the thin side of a badly worn piston. The thinning of the skirt is the result of wear but if yours was low time, not sure what could have caused that. Was the saw used a lot? Calender time is really no indication of use. If the saw was running 6 hours a day, it could wear out in a couple years. I suppose if the cylinder was "rough" or oversized, then that could cause abnormal wear as well.

Make sure you don't have any cylinder issues before redoing this saw again or you may be revisiting the same issue down the road...

I see in your original post you were estimating the saw was used only 10 hours after the rebuild. Was the piston used? I can't imagine any way the piston could get that thin in that few hours. Something doesn't make sense...

[ehp]

Check your base gasket , on the 044's there were a small amount of saws that the factory sent out with a metal base gasket that was the wrong size in bore , the gasket didnot have any wiggle room for when the piston got up to temperature , The piston grew a couple thousands from heat but the gasket did not and the result was the gasket torn the skirts off the piston and did it in a very short amount of hours . Stihl made the dealers change the gasket but I'm not saying all the dealers did change the gasket , If thye gasket is metal check it , if its not them check the intake port on the cylinder for something buggered up in it

[Al_Smith]

--either what Ed said or it ate something that didn't  agree with it and gave  it a terrible case of indigestion .

It doesn't take much,tramp metal orbiting around the crankcase ,ingested trash,a wire broken out of a flocked filter ,snag on the intake port .

The good thing is most OEM Stihl cylinders are pretty tough .I own an 038 Mag that literally grenaded a piston .I mean like a bomb went off .JJ from Oregon sent me an OEM piston ,back together and ran fine for 7-8 years until the owner retired it and gave it me .Never as much as put a dimple in that cylinder .

[ZeroJunk]

You could tell more if the broken pieces were intact . Still looks like it broke along a flaw in the casting to me. If there was damage from a gasket or piece of foreign material enough to break the skirt there would be damage on the bottom edge.

[lledwod]

OK thanks for the replies. I had a brainwave, duh, and measured the piston. The thin skirt seems to be due to wear. I will go through the saw more thoroughly in the next day or two, and try to track down the problem. So far the crank and bearings seem to be fine, I did split the crankcase and will clean everything up. I'll look the cylinder over very carefully as well though it seems fine. And I'll check out the carburetor, intake boot and filter for signs of a problem or debris. I suppose the damage could be due to me not cleaning out the crankcase properly last time I rebuilt it? I can't recall what I did at the time.

[ZeroJunk]

Ten hours or less must have been off a little.

[sablatnic]

Ten hours or less must have been off a little.

You didn't discard the new one and put in the old one? Don't start guessing why I ask!  :-[

Normally the skirt will break, when it is worn so thin, that the piston will start rocking, hammering the piston skirt into the cylinder wall. Normally caused by excessive wear.

[ktowne030311]

was just about to say, just because the cylinder looks good, doesnt mean its not ovaled out a bit, resulting in "piston slap". which would result in a broken skirt. measure the bore , see if its still factory spec.

[lledwod]

Yes I will measure the cylinder. I am kind of assuming it is nikasil plated, and assume this wouldn't wear egg shaped without wearing through the nikasil?

[AdkStihl]

The intake skirt looks worn extremely thin as compared to the exhaust skirt.
That's usually a sign of poor air filtration and/or beyond service life.

[ktowne030311]

Yes the nikasil can wear. The inside of the cylinder has to be exactly round ( with-in specs)
to perform properly. This can only be accurately checked with a dial bore gauge, it can measure to with-in
one ten thousandths of an inch. The Nikasil plating that's on the walls of your cylinder is only a  few
thousandths of an inch thick but it is very hard. It wears extremely well as long as everything is tight and
sanitary. If you maintain your engine as the factory recommends it will last for years.

[ehp]

problem is the bore is not in nicasil its in chrome which is alot softer , if the chrome is that bad you should be able to see with your eyes around the intake port cause if there is a problem that is where it will be . You better check your crankcase over very close , if any larger pieces got into the case with the motor still turning the crank can shove those pieces into the oil tank part of the case or straight down into the bottom part , either way check the casr out very well for any kind of cracks , to wear a piston skirt that bad something major is wrong , if you can see the shape of the intake port in the intake skirt on the piston then you got major clearance problem but kind of from the looks at the skirt I would say some bad stuff has been going by the air filter into the saw when its running , give us a shot of the intake side of the piston

[lledwod]

I took more photos, will post them when I get a chance. I measured the cylinder, I don't have the # in my head but I can detect no out of round. My method is telescoping gauge and micrometer. To 1/10 000". I'll test my memory, was it 1.9681" ......

I did split the crankcase, with the intention of careful inspection and cleaning. No real physical damage, though the paint is browning and flaking.



  

  

  

  

  

 

[Al_Smith]

Pistonoligy 101. Contrary to popular belief a piston skirt as a rule is not round nor perfectly square in relationship to the sides ,crown to skirt being parellel .Although I've shared this  with many saw builders we cannot agree .

Here it goes having installed two complete piston manufacturing lines .One  for Ford ,one for Honda and having done on going line maintance on 4 for Ford .

The crown or head if you prefer is perfectly round .The skirt is slightly larger than the face and has as slight ovality and a slight taper from bottom skirt to just below the piston rings .

The reason for this is as a piston is moving first it acts like a liniar sleeve bearing .However because of the forces involved it rocks slightly .With this rocking action because of the ovality the skirt presents a more perfect alignment with the cylinder than a perfectly round one .The ovality is measured at a right angle to the piston pin but because of the fact most modern saws use a reduced piston skirt it's not always easy to detect .

[Al_Smith]

More on this which involves my favorite subject with the amsoil crowd .Because in essence the piston skirt acts like a liniar bearing -plus a liniar valve it would make sense it requires lubrication

Now a 2 cycle engine doesn't have the luxury of being drowned with lube oil like a 4 cycle .Whatever lube it gets is only a microscopic film of the oil in the fuel .Heading right back to the ever popular oil wars .--see how I turned that one around  8)

[jwilly3879]

Pretty slick Al!

[lledwod]

 

  

  Thanks again everybody, for the info and ideas. I added photos to my post above. My measurement of the cylinder was 1.9681". There are two cracks in the rear housing where it is possible some debris could bypass the filter. Also, I did do some milling with this saw, with this piston. Personally I think I could re-use the cylinder, but I see chainsawr has a used piston/cylinder for sale, which is tempting. I will have to clean and inspect the crank bearings ....

[ehp]

that cylinder needs to be cleaned bad before even thinking about reusing it and you cannot measure the bore with it like that , to much build up on the bore , where is the second ring on the piston ? , A saw doesnot eat a piston like that unless something is wrong big time . I am quite surprised that the intake port part of the cylinder is not torn all to crap with a piston looking like that , I would of thought the cylinder would of had some pretty bad damage after that skirt dropped like that

[weimedog]

Just finished putting together a saw that had a skirt broken like that... and a chunk got down in the crank case and the crank punched it hard enough to make a hole in that case. Didn't notice it at first... just dumb luck as was using the parts cleaner to flush out the cases and noticed leaking where I didn't expect.... closer inspection. Ruined cases.

[Al_Smith]

On a grenaded piston it's good advice to completely field strip the saw clear down to the bones .

That black stuff inside the crankcase is most likely very fine wood dust which combined with the oil from the fuel has became pyrolited .In essence turning it to petrolium coke .The oil didn't do it ,the dust did .

You can go less expensive with say piston rings but a good filtration system is a must .

[HolmenTree]

lledwod, I see your up in the Yukon and probably run your 044 in snow and freezing cold conditions.
Snow ingestion which will wash away the lubricant off the piston intake skirt causes damage  called water or snow score .
Adding alcohol de-icer to the fuel mix to prevent the icing up condition in the carb's venturi, will further add to washing away the piston skirts lubricant film.
Under these conditions  with no lubrication film the piston's intake skirt will wear thin very quickly as your photo shows. Plus on top of that if the saw is icing up it will be running lean furthering the problem. Piston rocks severely slapping the cylinder breaking apart the piston skirt.

My other question is was the saw run without a lower piston ring to gain extra r.p.m.? If it was that could be another reason for the piston rocking causing excessive wear.