Stihl 021 - Burnt Clutch & Housing

[SteveSr]

Hello,

The first time I got this saw the clutch and the surrounding had been overheated and burnt around the edges. The worm gear was also melted. I cleaned up the plastic as best I could and replaced the worm gear and sent the saw back to the volunteer user.

Got the same saw back in on a saw swap today and saw the chain hanging loose. Chain wouldn't pull. Pulled the clutch cover and found more burned plastic and clutch bearing melted. Haven't pulled the clutch yet.

The first initial damage was caused by poor running (old gas) & improper user technique. The second time the saw was running properly so am wondering how much of the second instance is due to poor user technique or if the initial damage closed clearances which allowed oil and sawdust to collect and eventually cause the brake band to rub on the clutch causing the second instance.

My next question is how to fix this if possible. At this point I am tempted to take a Dremel to open up the clearances in the plastic around the brake band (without taking out the brake band supports). Both the clutch drum and clutch are discolored from heat. Are these parts reusable assuming that the clutch springs are still good (clutch doesn't engage at idle).

Your thoughts?

Thanks,

Steve

 

[SEAM]

Keeping the chain sharp helps...
The Stihl design with the inboard clutch makes it difficult to dissipate excess heat. A sharp chain and ample bar oil will help to prevent overheating.
Also, if one of the rotating parts (clutch, worm gear etc.) contacts the housing or the brake band contacts the clutch there will be excess heat.

 

[HarleyT]

Likely the initial damage was caused by someone trying to run the saw with the brake on,

put up some pics.

 

[Definitive Dave]

Tell them to turn the brake off when running the saw, likely the problem both times from my vantage point on this side of the screen

 

[SteveSr]

Likely the initial damage was caused by someone trying to run the saw with the brake on,
put up some pics.

... Or leaving the saw in fast idle (starting position) with the chain brake on.

Pictures on the way...

 

[SteveSr]

We all know how much Harley likes pictures... Well here you go!

Warning for those with serious CAD these pictures of chainsaw carnage are pretty UGLY!

For the picture with the clutch drum in place I added a replacement bearing to provide correct spacing of the drum.

 

[HarleyT]

With the sprocket off, engage the brake and take a pic, to see if any molten plastic is pushing the brake band inward.

 

[Chris-PA]

Yet another melted Stihl.

Plastic Stihl?

 

[SteveSr]

With the sprocket off, engage the brake and take a pic, to see if any molten plastic is pushing the brake band inward.

The first pic shows the brake band engaged. The last pic shows the brake band disengaged.

I can't see any places where the plastic is pushing the brake band into the clutch drum. My main concern is the close to zero clearance between the clutch drum and the surrounding plastic. It wouldn't take much packed sawdust behind the clutch band to push it into the clutch drum.

 

[Stihlofadeal64]

We all know how much Harley likes pictures... Well here you go!

Warning for those with serious CAD these pictures of chainsaw carnage are pretty UGLY!

For the picture with the clutch drum in place I added a replacement bearing to provide correct spacing of the drum.

View attachment 477750 View attachment 477751 View attachment 477753 View attachment 477755

Seen this many times with saws in this series (021, 025, even the MS250 etc). These saws can be ran with the chain brake engaged
and it will produce this problem (as already stated). One of the backlashes that is seen is that eventually it toasts the case so it cannot
be reused, and/or the oiler ceases to work.

Your attempt to clean up what you can is a worthy endeavor if you don't mind the extra effort. I'd check the brake band as well. It may
be damaged at this point as well and need replacing. I have also replaced the dust cover for the chain brake when going over one of these.

 

[SteveSr]

Seen this many times with saws in this series (021, 025, even the MS250 etc). These saws can be ran with the chain brake engaged and it will produce this problem (as already stated). One of the backlashes that is seen is that eventually it toasts the case so it cannot be reused, and/or the oiler ceases to work.

Your attempt to clean up what you can is a worthy endeavor if you don't mind the extra effort. I'd check the brake band as well. It may be damaged at this point as well and need replacing. I have also replaced the dust cover for the chain brake when going over one of these.

Thanks for the feedback. After grinding some burnt plastic I noticed that unlike other Stihl saws there appears to be nowhere for any of the accumulated oil/sawdust to go. Other saws have "escape" holes in the crankcase to allow this detritus to get out of the clutch area. This one there is nothing. Are all of these plastic saws like this or did Stihl eventually fix this oversight?

I am assuming that under this situation eventually enough junk gets behind the brake band to push it into the clutch drum and start the overheating.

 

[HarleyT]

I have never seen that......

 

[SteveSr]

I have never seen that......

So all of you money is on 100% user error x 2?

Well, If I can make it go again I'll run it a little while to make sure there are no other obvious problems. If I don't see any more problems it'll go to a different volunteer. Anyone want to place their bets now?

 

[a. palmer jr.]

I had one like that but not that bad and I just took a pocket knife and trimmed the plastic around the edge. They sell new aftermarket cases for that saw for just a little money. Check ebay..

 

[Stihlofadeal64]

If you decided to replace the case, that's not a bad idea. I don't know that it is cost effective if inexperienced volunteers are going to run the saw again. I really can't say what your options are not know the philosophy of those who are pouring green backs into your effort.

 

[AKDoug]

This video I made shows how little time it takes at high idle, with the chain brake on, to heat up a clutch.

 

[SteveSr]

I put the saw back together and it runs and cuts. However I have no confidence in its long term reliability without replacing the case. The plastic has ribs that are used to center the brake band around the clutch drum. With these melted away the band is way too close to the drum at the top where the brake band sections cross. It won't take much for the band to hit the drum and start the whole melting process all over again.

 

[a. palmer jr.]

It appears there's a new aftermarket one on ebay for about $33. I've never tried one so don't know how good they are.

 

[computeruser]

My thoughts: the volunteers need proper training and/or to run their own saws.

 

[Stihlofadeal64]

My thoughts: the volunteers need proper training and/or to run their own saws.

+1.