Stihl 039 Teardown

[hms11]

Hi Everyone,

After tearing down my old 039, and taking enough pictures that I hopefully don't get lost when re-assembling, I thought it would probably be a good idea to put up a post with a bunch of pictures that can maybe help others if they get into a teardown of one of these clamshell saws.

First up, cover removals. I'll try and keep the number of pics per post down to help with loading the post.



 
 Here is the back cover and air filter removed. From what I can tell, Stihl didn't add the compensator to these saws until the MS390

 
Here is before and after the clutch drum has been removed. I have also removed the saw brake cover at this point.

 

[hms11]

Next up, chainbrake and clutch tear down! I probably got a bit picture heavy with the chain brake but there were enough little pieces in there I didn't want to lose track of them.


  

 When removing the clutch, use a rope through the plug hole and rotate the engine until it binds up. The clutch is on a reverse thread (righty loosey, lefty tightey), good to keep in mind.

 

[hms11]

Flywheel and Coil area:


 
 I didn't have a flywheel puller, but the old tried and trued method I've used for tapered shafts in the past worked well. Loosen the nut but leave it threaded on a bit. Get a centre punch a give it a firm wack in the centre of the crank with a hammer. With a solid wack or two you should feel the flywheel knock off its taper. At this point, remove the nut and lift the flywheel off.

[hms11]

Getting down to the bare bones:


 

Not much to say here, just taking your time, removing bolt on shrouds, carb boots and oil lines. I like to give old rubber lines a bit of a working through with my hands as I take them off. It gives me the opportunity to find cracks, splits or other damage that might not be obvious without manipulating the hose. I'll be getting a new Stihl fuel line for this saw.

 

 

 Considering this is the first time this saw has ever been apart (to the best of my knoweledge) it doesn't look THAT bad for its age. Dirty, caked in sawdust and oil but nothing obviously worn out or broken (other than the piston that it got torn down for originally).

[hms11]

The clamshell emerges!


 

 As we can see, I misdiagnosed this saw originally, but the level of carbon in there makes me glad I tore it down anyways. When the saw was together, the machine hole in the center of the piston looked like a burn mark. This, combined with the ridiculous level of carbon build up led me to tear the saw down. I'm going to put a new piston and rings in while I've got it town down, along with crank seals, and possibly a new carb to be able to take advantage of the carburetor port of my new airfilter.  

  

[Fishnuts2]

Nice pics, but how did you get that rotten bar stud out?  The last one I did, I left the pan and the stud in the plastic chassis.  Saved a whole lot of aggravation.

[hms11]

The bar stud was..... a treat. Stihl puts a bit of a threadlocker on there during installation, after 20+ years, it had quite a grip. 

I ended up using a combination of doubling up the bar nuts and using a pair of vicegrips with a rag to protect the shaft where the bar sits. The vice grips gave me the slightest bit extra grip and I got the threadlocker to break loose. After that it was same as any other stud and the double nut setup did fine. 

I was considering leaving the pan in the chassis, but I just couldn't bring myself to do it. I wasn't going to be able to clean and inspect everything to my satisfaction with that pan sitting in there. 

[Fishnuts2]

I've done 4 or 5 of those, and each stud comes out differently.  I always try the double nut deal, but the stud twisted off on the first one I did, so I had to use vise grips a little at a time to get the remainder out.  I also have a stud puller that uses a cam against the stud to gain grip on it.  Whatever works, I guess.
 
Leaving the pan in place works just fine if you clean the whole area before taking it apart. 

[moodnacreek]

Have torn down 2 039's all the way. I start with leaving the power head in a pail of gas and then about an hour with a blow gun. Anyhow the newer one will only run right for 10 min. just like it did before. Crank seals, coil, all rubber parts and 3 different carbs [tested on other saw] . This 039 is like new but only good for a few cuts. The old worn one [with new jug] is very reliable like it always was.     What am I not seeing?

[lxskllr]

Have torn down 2 039's all the way. I start with leaving the power head in a pail of gas and then about an hour with a blow gun. Anyhow the newer one will only run right for 10 min. just like it did before. Crank seals, coil, all rubber parts and 3 different carbs [tested on other saw] . This 039 is like new but only good for a few cuts. The old worn one [with new jug] is very reliable like it always was.     What am I not seeing?

Sounds temperature related. Using a coil that's known to work on another saw? Maybe something else affected by temperature?

[hms11]

I've done 4 or 5 of those, and each stud comes out differently.  I always try the double nut deal, but the stud twisted off on the first one I did, so I had to use vise grips a little at a time to get the remainder out.  I also have a stud puller that uses a cam against the stud to gain grip on it.  Whatever works, I guess.

Leaving the pan in place works just fine if you clean the whole area before taking it apart.

Seeing how much of a fight I had with it, and the negligible benefit you get removing the pan and stud from the chassis, I'll likely leave it in next time. I have a bit of a bad habit of taking fixes, rebuilds, etc far further than they need to go. 

[hms11]

Have torn down 2 039's all the way. I start with leaving the power head in a pail of gas and then about an hour with a blow gun. Anyhow the newer one will only run right for 10 min. just like it did before. Crank seals, coil, all rubber parts and 3 different carbs [tested on other saw] . This 039 is like new but only good for a few cuts. The old worn one [with new jug] is very reliable like it always was.     What am I not seeing?

Sounds temperature related. Using a coil that's known to work on another saw? Maybe something else affected by temperature?

I agree, sure sounds like the coil is getting hot and breaking down.
The other thing I could think of is if you are in a hot climate and that saw is in "winter mode". Once the saw gets heat soaked the carb might be acting up with all that hot air being blown through and past it. My saws only ever sit in winter mode, but I live in Canada and pretty much only cut in the cooler seasons. 

[moodnacreek]

Thanks for the suggestions. Tried different carbs and coils the ran on other saw. Dealer says only thing to do is pressure test. That is a pain in the........ I guess some day I should force myself to do it.

[hms11]

One last suggestion, gas vent line? Is it possible the fuel tank vent is plugged. Tanks that can't vent right cause all sorts of weird problems, usually after some running for a while too. 

[moodnacreek]

One last suggestion, gas vent line? Is it possible the fuel tank vent is plugged. Tanks that can't vent right cause all sorts of weird problems, usually after some running for a while too.

Good point. As I remember the vent was off it , and leaking, when all this started.

[Fishnuts2]

@hms11
Looking over your pictures I noticed that the shutter for Winter/Summer use was turned for cold weather use.  Maybe that's on purpose for this time of year, but a lot of the 029's and 039's I get or work on have them like this all year round.  Plus, they have the black screen or divider above the air filter set in the wrong position, or missing completely.  Seems that the manufactures all warn about not running a saw with shutters set in the wrong position for temps above 32 degrees.  Obviously, your saw was not harmed from it, but it might make for interesting discussions like does it help or hurt efforts to keep the air filter from getting dirty so fast, etc.

Just my thoughts

[hms11]

@Fishnuts2 

Every saw I've ever run has lived its entire life in "Winter Mode", unless I have to do the odd bit of chainsawing in the summer. 

Living in Ontario, and doing 99% of my chainsawing from October to April-ish means my saws only ever work in single digit temperatures (C, not F) and below. I was never sure exactly where the cross over from running "summer" to "winter" setups was on the saws but seeing as mine always run from Fall to Spring I figured I was safe just leaving them in winter mode. 

Do you think its a good idea to put it in summer mode even if its 1-9 degrees Celsius outside? I can switch it over, it doesn't take long, just didn't seem needed in the cool temperatures I typically cut in.  

[Fishnuts2]

I guess if you run mainly in the cooler months it probably doesn't make much difference if you run during a warmer day once in a while.  Currently, it's a -5 C here, and none of my stuff is set for Winter.  The coldest I tried was last year at a charity cut reading  at a -20 C.  All four saws ran fine, only needing a little richer carb setting, and no carb icing.

[hms11]

Well now you have me curious.....

Maybe when I put it back together I'll switch it to summer mode and see if I notice anything. 

[hms11]

The reassembly has begun!
After a liberal soaking in the parts cleaner, everything is clean, shiny and ready to be put back together. 



 

The Stihl dealers local to me gave me a price of $233 for a piston with rings. I just couldn't bring myself to spend that kind of coin on a piston so I did what many of you will likely find sacrilege. I put a Cross Performance piston in it. I couldn't find anyone locally who ordered Meteor pistons and of all the aftermarkets that AREN'T Meteor, Cross seems to be at the top. On the plus side, they come with Caber rings. 




 

I put a new genuine Stihl fuel line in the chassis, I had put a small nick in the carb end when disassembling and had heard horror stories of the aftermarket ones kinking. $7 at the dealer, hard to go wrong. 



 

Piston and rings installed. I reused the OEM circlips. The Cross ones had large ears on them and in my experience in 4 stroke engines, having extra, non-needed material in the rotating components, especially the piston is asking for future failure. OEM clips were "springier" as well, and locked in the groove very positively. 



 



 

Back in the chassis! Getting the piston into the cylinder wasn't bad, just take your time, be careful and don't force anything. Stihl puts a nice taper on the bottom of the cylinder so you can generally ease everything in there, taking time to help a ring by a transfer port edge every now and then. I put the pan in the chassis and threaded the rear bar stud in lightly by hand to hold it generally in place. Then I put a bead of Three Bond around the pan with a light smear where the seals the ride. I also put a smear on the seal area on the cylinder side. Once I managed to get the cylinder with rotating assembly into the chassis with the pan (hindsight being what it is, I see why guys use spacers on the bolts and put the engine together outside of the case first, that's a sneaky fit!) I used a cross pattern to tighten down the pan. I removed the bar stud for pictures as it was only in there for locating reasons anyways. I also put a smear of Three Bond on the outside of the crank seals, where they meet the engine, I've seen this mentioned as a good idea multiple times when reassembling these motors.

Now I'll be waiting 24 hours, and putting the rest back together!

[Fishnuts2]

Looks like you'll have a like new saw when done.  Cross also makes a piston for 039's that has a pop up on the piston to increase compression without other mods.  It would have been interesting to see a compression test of the old setup with all that carbon on the top of the piston vs the new standard piston.  Thinking that it would have been close to what a pop up would have done for you!

[hms11]

I kept randomly coming across the Cross pop-up, but I couldn't find any Canadian sellers with one. I was seriously considering putting one in the saw because.... why not? I've spent far more time and money on this saw than I should already, whats another $10-20 over top of the "stock" Cross piston?

But, I couldn't find one, so I went with the standard compression piston. The piston did weigh out to be a couple grams lighter than the stock piston, so I might pick up a bit of performance just in the lowered rotating weight, it also seems to have nicer windows in as well so we will see. I was actually fairly impressed with the piston when I compared it to the Stihl, it looks and feels like a quality piece. 

The saw should basically be brand new when I'm done. I decided on a new carb as well so I could take advantage of the more modern compensator style air filter. 

Is it worth it? Definitely not by any rational measure. Will my grandfathers old saw be ready for another 20+ years of work? Yup!

[hms11]

Flywheel and coil area reassembled. Some tight areas around the wiring for the coil and plug. Filled the combustion area with starter rope to lock the engine from rotating in order to tighten down the flywheel.


 
PTO side with oil pump reinstalled.


 
Carb boots, wiring, impulse and fuel line routing and all the little spacers for the carb/boot mating area. Orange cover back on with AV mounts and handle reattached as well.



Front view. It's a wise idea to install this muffler stud BEFORE you put the white cowl over the motor. If you forget you can wedge it in there with some patience and a flat heat screwdriver, but it's way easier to just do it beforehand.
 

[hms11]

Muffler mod details. Opened up the top two slots substantially and also added 2 additional 1/4" holes. Opened up the mouth of the deflector as well but left enough to keep it shooting out and to the right, over the bar. I didn't want to drill into the depression area as that bypasses the internal baffle and I just want to wake the saw up a bit, not make it annoyingly loud. 


 
Chainbrake and clutch all cleaned up and reinstalled. Make sure to clean and re-grease the needle roller bearing for the clutch drum. 


 
Carb area mostly back together. I don't have the winter/summer deflector or airfilter back on yet in this pic. I ended up just taking the compensator style top off the new carb. The aftermarket carb wasn't filling me with confidence so I just stole the top and diaphragm off the new carb, cleaned up the original Walbro and put it back on.


 
Clutch drum, washer and clip back on!



All back together and waiting on wood! Still needs the carb set up and the engine broken in. What do you guys typically do for breaking new saws in?
 

[lxskllr]

Saw looks great. Nice job!

For break in, I take it easy on the saw. Use a range of rpm cutting, and not a long time running full out. Limbing would be a good activity imo.