Author Topic: Oil seals on Stihl MS660 keep going bad.  (Read 201 times)

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Offline emhusq66

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Oil seals on Stihl MS660 keep going bad.
« on: December 30, 2019, 05:49:36 am »
Hello Everyone, I am new here and this is my first question. I picked up a Stihl MS660 at a estate sale (not running), I did some research in the service manual and it says "saw is hard to start, will not idle, runs fine in WOT". That is exactly what it is doing, so it says to replace oil seals, which I have done now 3 times. When I start it after replacing the oil seals, it starts on 2 pulls and runs great for awhile, then when I try to start it again, same thing- hard to start, etc... I don't understand what I am doing wrong (if anything). I did a leak down test and it was still holding pressure after 20 min. I really need this saw to run but I'm lost as what to do, any help would be greatly appreciated, Thank you.

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Offline Cut4fun .

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #1 on: December 30, 2019, 07:43:56 am »
Have you checked impulse line and replaced if old?

Had a 66 come in pretty much sounded like this and was just impulse line.

Have you checked your crank bearings to make sure keepers on both sides when doing seals?

Just making WAG's over net.
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Offline emhusq66

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #2 on: December 30, 2019, 09:31:45 am »
First thank you for getting back to me so quickly I might check the impluse line regardless of the new parts. The impulse line is brand new and so are the main bearings which I put in myself. Pretty much I tore the whole thing down and replaced everything I could, I also added the big bore kit (56mm) and was planning on buying a Hyway pop-up piston for the bigbore until all this happened.
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Offline 3000 FPS

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #3 on: December 30, 2019, 06:27:52 pm »
When you say oil seals are you referring to the crank seals.
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Offline emhusq66

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #4 on: January 01, 2020, 02:15:09 pm »
Yes Sir, the crank seals. Sorry about the delay.

Offline 3000 FPS

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #5 on: January 02, 2020, 12:45:25 am »
If the crank is smooth where the seal rides on it and the seals are the correct ones then I do not see how they would go bad.   Plus you said you did a pressure test on crank case and it holds pressure for 20 minutes.   I do not believe the seals are your problem.   I would be looking for a air leak some place else like cut4fun mentioned.  The impulse or the intake boot and I would even check the fuel line.   Heck I would even suspect the carb as possible problem too.   Have you tried adjusting the carb to see if you get different results. 

When tuning a saw it can sure act different when it is hot as apposed to when it is cold. 
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Offline Al Smith

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #6 on: January 02, 2020, 08:43:37 am »
Just as a general statement often times the only sure way to determine if a boot is torn is to remove it because often they get torn on the bottom side .Which is often times caused by worn out anti vibration mounts allowing the engine moving too much within the suspension system .
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Offline Cut4fun .

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #7 on: January 02, 2020, 09:42:59 am »
I've seen them torn under the clamp
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Offline emhusq66

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #8 on: January 02, 2020, 06:06:26 pm »
Thanks Guys for your responses, i'm definatly going to check impulse line, intake boot, and carberator. I will get back with the results by monday, (I've got a busy weekend :o) I'll be in touch and thanks again.
                                                                                                                                                                                                                                                                                                    emhusq66
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Offline emhusq66

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Re: Oil seals on Stihl MS660 keep going bad.
« Reply #9 on: January 24, 2020, 10:37:35 pm »
Hey guys, I figured i would finish this story so if anyone else is having this problem maybe this will help. First I would like to thank cut4fun and 3000fps, for their information they hit it on the head, The culprit was the intake manifold with a 1/4" slit on the bottom. The saw runs like a champ now , what a beast! One more time, Thanks too cut4fun and 3000 fps for their time and input, this site is is awesome. God Bless.
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