ms 660 air leaks

[jpryan]

1st post (new Member) Hello my handle is Jim and I have had a recurrent problem with the Stihl Ms 66o. I have a logger friend who logs firewood 9 cords/day 6 days a week all year. a saw lasts him about 14 months and then its a parts saw.  All the failures can be traced to air leaks.  Of 33 parts saws I was  able to rebuild 27 of them. The other 6 remained parts saws.  Two had cracked cases and the other 4 had a leak around the chain side bearing seat and through the oil pump mounting screw holes (most discouraging).
I have not seen a post in this forum on that problem and I cannot imagine a reliable fix for it.  The leaks around the bearing seats is quite large (cannot pump past 5 lbs and it drops off in a second or two to zero. A side note I have found that using a piece of SS shim stock (.003) around the crankshaft when replacing the seal will keep the spring in the seal from dis-logging when going over the step on the shaft.
I cannot imagine too many people have gone through 33 saws so it does not surprise me that this problem has not shown up on the forum. Have a nice day Jim
   

[Lorenzo]

Judging by the response, or lack of, I hope this is not a common issue because I own four of these,
One 066 Red light and three MS 660's

I personally haven't had this happen or even heard of it till now but I am not a commercial forester and don't use them every day. I hope you can figure this out and post a resolution, also whats causing it so I can try to avoid it happening to me. Do you have a theory ?

[jpryan]

first thanks for your reply.  I thought at first that the bearing seat was pressed in, I had to cut the chain side case in half to find that the seat is cast in, the casting covers both lips of the seat and one would think that no air could get past.  There is a problem though with the mounting screw holes for the pump, they are so close to the seat that the threads cut into the seat creating a path for air to circulate around the seat and also into the the holes. I am currently trying to correct this leak by pulling a vacuum on the case and sucking thread lock tight around the seat and in the screw holes.  So far I have been able to pull a 10 lb vacuum for maybe 5 seconds where as before it was not possible to pull any vacuum I will let this harden up for 24 hours and see if I can suck any more locktight between the seat and case.  Being that you have 3 of these saws a pressure/vacuum tester is a great investment. Jim

[jpryan]

This is an update on the first post of 660 air leaks.  In my first post I misstated the number of machines that had this problem.  My own 660 also leaks around the bearing seat and pump screw holes and will not hold .5 bar (7.25 psi).  Pulling a vacuum and sucking lock tight through the screw holes and around the bearing seat and letting it set up for 24 hours was successful.  I am now able to hold .5 bar indefinitely, the specs. call for 20 seconds. This tells me that if everything else is tight any leakage past 20 seconds is around the bearing seat.  I hope this is helpful to anyone that may have similar problems. Jim

[weimedog]

It is an interesting detail that can explain why some 660's run better than others... most have no idea and just twist on the carb adjusters to compensate. But you just articulated a trade "secret" for some...to get better case vacuum numbers therefore better consistency from saw to saw...

[weimedog]

I have a few cases to check...looking for the issue.

https://forestryforum.com/gallery/albums/userpics/11485/20150714_1814575B15D.jpg

https://forestryforum.com/gallery/albums/userpics/11485/20150718_1154165B15D.jpg

https://forestryforum.com/gallery/albums/userpics/11485/20150718_1154045B15D.jpg

[jpryan]

thanks for your reply in my case about 14% of the 35 saws had leakage around the bearing seat that I noticed. In all cases I was looking for seal leakage.  The case I cut apart, the seat remained in the one half and you could see a gap between the magnesium and the seat, the gap would accept a .001 shim stock sliver but not a .002 sliver to about 1/2 inch in depth I would guess that the clearance was about .0025.  the bubbles were not 100% around the seat some spots would not draw in the locktite with a vacuum.  Some of the saws that passed the 20 second test may well have had some leakage around the seat that I did not notice since I was applying the leak test liquid to the seal only.  The crankcase side of the seat has gear like teeth that are much deeper than a knurl which effectively locks in the seat.  And  I am still amazed that so much air got past (that 1 lb of pressure or vacuum was impossible to achieve with my hand pump.)  So my experience tells me that one should check all around that area the fix is dirt cheap. Jim

[jpryan]

I rebuilt another 660 today and while the pressure tests were spec. ok there was air leakage in one of the pump screw holes but none around the bearing seat.  It will be interesting to see if the leak progresses to the top of the seat. (The pump screw normally seals the leak off and if it does not a little locktite will.  I keep a log of all the saw serial numbers and their problems and fixes. Jim

[Lorenzo]

Has there been any issue regarding these air leaks on an old 066 model Stihl?

Or ,are you only seeing this on the newer 660?

[weimedog]

The reason I posted those pictures it I don't see where the bearing seat intersects with the oil pump mounting screws AND the screw holes don't go all the way into the crank case volume like Husqvarna 372's do. I put the 372 screws in with Three Bond 1194/84 to seal them. If the cases are leaking around the bearings....are those bearing pockets damaged from wear & tear?? Cases flexed? Lousy machining & alignment?

Sounds like you were thinking porosity in the casting & machining ...

Those pics I posted were Huztl/Farmertec  cases BTW. Casting quality was actually quite good. AND I agree with your premise sealing around the bearing seats is cheap and might be a simple procedure that makes that saw run much better...

Would be nice if you could post pics or a vid

[lumberjack48]

 Only 14 months, something is causing the seal to go, or is it only the 660 having this problem. This is usually cased from running the chain to tight.
I had a 038 Mag that bucked 10 to 40 plus cds, 5, 6 days a week for 8 yrs, never had a seal problem.
I had a 044 Mag we ran 8 yrs, felling and bucking, never had any issues with it.
The first 034 AV i had, i fell over 12,000 cds before any issues with it.
Its really hard to understand that, that many saws had the same problem, i would be looking for the problem thats causing premature failure. Another thing that could cause this is, running rakers to low and or pinching the saw alot and pulling it up and down to get it free.

[jpryan]

Lorenzo one of the saws that I parted out for this air leakage around the seat and into the pump mounting holes was an 066 SN 11924---- the other 4 were MS 660's

[Dixon700]

Hmm what's the tooling and setup all you guys use for checking for vacuum leaks? N I think I want to remove my clutch and put loctite on my oil pump screws since I recently replaced oil pump out.