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Stihl MS660 big bore conversion.

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Old 10-08-2015, 03:33 PM
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Jim.Thompson
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Default Stihl MS660 big bore conversion.

I've just been given a 66 cc Stihl big bore engine, still in the saw. The saw is a MS660, if that helps with engine identification.
I am in the process of stripping all the saw parts off it and plan to convert it for aero application. I plan to build a tug plane for towing thermal gliders. I will probably open a separate thread about the design and build of the plane.
I've searched this thread and could not find a thread describing a conversion of this model engine. Has anyone done one of these?
If not, any tips about similar conversions will be most welcome.
This engine has the magneto mounted on the plastic saw body, so a conversion to CD ignition with a machined prop drive shaft is indicated. It will also result in less weight, another advantage for a plane motor.

I have read a cautionary note about cutting the alloy on these saws as they are (allegedly), magnesium alloy and it may be dangerous to do so. Can someone validate or dismiss this and clarify with all precautionary details please?

Jim.

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Old 10-09-2015, 01:17 AM
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Hi Jim,

One, you might already know this, but never use grinding stones on magnesium or aluminum. They will quickly clog up, then will overheat and can explode. Use carbide cutters only. Then as far as magnesium itself, yes you must be careful. Magnesium grindings or shavings can be accidentally set on fire by a spark, and it burns white hot with a fire that cannot be put out with water. So you need to clean up often and safely dispose of it. If you do not let too much accumulate at a time, you'll be fine.

Here is my post and pics of the Stihl 62cc that I converted.
Stihl 62cc challenge....

AV8TOR
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Old 10-09-2015, 04:12 AM
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A Stihl MS660 is 92cc from the factory and the big bore cylinder & piston makes it 99cc
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Old 10-09-2015, 09:14 AM
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Originally Posted by w8ye View Post
A Stihl MS660 is 92cc from the factory and the big bore cylinder & piston makes it 99cc
Yupp it's true I read it on the internet .. It would truly be a shame to turn a 1,000.00 saw into an airplane engine , but if a sacrifice has to be made it it might as well be for an airplane .
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Old 10-09-2015, 12:32 PM
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Originally Posted by av8tor1977 View Post
Hi Jim,

One, you might already know this, but never use grinding stones on magnesium or aluminum. They will quickly clog up, then will overheat and can explode. Use carbide cutters only. Then as far as magnesium itself, yes you must be careful. Magnesium grindings or shavings can be accidentally set on fire by a spark, and it burns white hot with a fire that cannot be put out with water. So you need to clean up often and safely dispose of it. If you do not let too much accumulate at a time, you'll be fine.

Here is my post and pics of the Stihl 62cc that I converted.
Stihl 62cc challenge....

AV8TOR
I did not know that about the magnesium cutting precautions, many thanks for that. I will have to chase up some carbide cutters and a suitable power tool to do the job. Maybe a carbide blade for my little 4 inch grinder is available? I only have a carbide blade for a 71/4 in power saw, which is far too big.
Thanks for the link to your thread above. I had seen it, but not studied it closely enough. The principle will be very similar. I was a bit stumped by the challenge of making up an intake manifold adapter; your idea is neat and simple enough for me to copy.
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Old 10-09-2015, 12:36 PM
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Originally Posted by w8ye View Post
A Stihl MS660 is 92cc from the factory and the big bore cylinder & piston makes it 99cc
Thanks for that clarification. Wow! The plane I have plans to design for it is now one order larger.
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Old 10-10-2015, 06:03 AM
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There used to be? an outfit in Wa. state that did a lot of hop-up work on Stihl professional saws. They once worked one over for me and it worked out well, it had to do with porting as far as I know? Much stronger
torque and rpms much increased. It was not at the expense of reliability as I ran that saw for quite a long time and never had a problem. Sadly I don't remember their name or if they are still in business.
The work was done thru a friends saw shop........
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Old 10-10-2015, 12:57 PM
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Gizmo.
I don't think this engine will need any mods to improve power; it already develops around 7 bhp in standard form. If I think it needed it, I would do the mods myself as, a) I live in Australia and b), I am able to do it with access to machining and milling equipment and advice etc. I'm a veteran DIY type, with a wide range of experience.
Besides, Av8tor describes modifications of similar engines very well on other threads here on these forums.
Thanks anyway. It's good to know that the engine mod hobby and industry is so alive and active in your country.
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Old 10-10-2015, 03:08 PM
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As a saw they are not lacking for sure and will run a 36-38 inch bar with no problem..........I always ran Stihl Syn at 50 to 1 the reccommended mix and never had a problem on 100 plus degree days running hard.
Keep us posted.
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Old 10-10-2015, 05:35 PM
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Originally Posted by Gizmo-RCU View Post
...................................
Keep us posted.
Will do. Currently, a mechanic pal of mine and me have been unable to remove the clutch assembly. We are assuming the retaining nut is a left hand thread. Even a big rattle gun is not budging it!
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Old 10-11-2015, 05:01 AM
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The clutch is left handed
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Old 10-11-2015, 06:48 AM
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Put a piece of soft rope in the spark-plug hole leaving some out for later removal (after removing the plug) turn up against the piston totally stopping it, use a drift punch on the clutch, there should be several raised casted lugs on the main body for this purpose. Just a sharp rap on the drift should do the trick. This is how most saw shops do it! Clutches and sprockets require maintainance fairly often and I have done this a number of times over the years. Very easy after doing it a few times.........
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Old 10-11-2015, 12:11 PM
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We got the clutch assembly off. It required a bit of heat and a big rattle gun. Very tight.
Before dissembly of the crankcase, we noticed the shaft could be moved up and down about 250 microns or so! The slop turned out to me between a sleeve an the alloy casting around the output bearing. We don't know why the alloy has a 2mm wall thickness sleeve here, it is certainly original. But the problem is between the sleeve and the surrounding casting.
The main bearing will be replaced. However, the output bearing looks like a special Stihl part. I will check to see if there is an after market replacement.
I'm wondering if common sealed bearings will servo ok in a 2 stroke crankcase and take the crankcase compression. Anyone know? I seem to recall seeing plain sealed bearings in other two strokes like mowers etc.

edit:
I have pictures to post, but I don't know how to do so on these forums yet.
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Old 10-11-2015, 03:13 PM
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the case does need to be sealed well and standard contact seal bearings don't always cut it. only reliable way to use these is to leave both seals in place and rely on the grease for lube..... the back end (clutch side) can simply be sealed off with a plate or with whatever you cook up for a mount....but only if you cut the unused part of the crank off. front side can also have a plate made to hold a standard oil seal that will do the job just fine. pretty sure you're going to need something to retain the bearings anyways. and the case is most definitely magnesium. my favorite tool for doing the rough cutting is a good bosch jigsaw with bimetal aluminium cutting blades. a good band saw would probably be even better. save all the scraps for a bonfire...
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Old 10-11-2015, 06:50 PM
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to post pictures, you are given the opportunity by using the "Go Advanced" option in the lower right corner.

You can also use the insert image picture icon in the quick reply editor.

In both these you will need the picture on your hard drive.

You can use a host like Photobucket for your pictures and link to them via the "img" option.
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Old 10-11-2015, 06:58 PM
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Originally Posted by ZAGNUT View Post
the case does need to be sealed well and standard contact seal bearings don't always cut it. only reliable way to use these is to leave both seals in place and rely on the grease for lube..... the back end (clutch side) can simply be sealed off with a plate or with whatever you cook up for a mount....but only if you cut the unused part of the crank off. front side can also have a plate made to hold a standard oil seal that will do the job just fine. pretty sure you're going to need something to retain the bearings anyways. and the case is most definitely magnesium. my favorite tool for doing the rough cutting is a good bosch jigsaw with bimetal aluminium cutting blades. a good band saw would probably be even better. save all the scraps for a bonfire...
Excellent information Zagnut; just what I was going to ask today. The treatment of the main bearing seals you describe is very good.

Originally Posted by w8ye View Post
to post pictures, you are given the opportunity by using the "Go Advanced" option in the lower right corner.

You can also use the insert image picture icon in the quick reply editor.

In both these you will need the picture on your hard drive.

You can use a host like Photobucket for your pictures and link to them via the "img" option.
Thanks for that. I will give it a go now. (maybe later, my line speed is slow just now).
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Old 10-11-2015, 07:22 PM
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Originally Posted by ZAGNUT View Post
the case does need to be sealed well and standard contact seal bearings don't always cut it. only reliable way to use these is to leave both seals in place and rely on the grease for lube.........................................
Are standard pre-packed and sealed bearings rated to the revs we are interested in here? The Stihl will develop it's max power at around 9000 rpm.
However, I am more interested in the open bearing option combined with an external seal and housing. The shaft appears to be plenty long enough and it would be a simple machining job.
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Old 10-11-2015, 10:17 PM
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i'm guessing the bearing size would be a 6203 or 17mmx40mm....if so then the open type using oil is good for 21k and the standard contact seal with grease for 12k rpm. if you're up to it then using a real seal is the right way to go. that some manufacturers of high end "made for RC" gas engines use work-arounds like sealed bearings and relying solely on glue to locate bearings does not make it good engineering practice....when these things go bad (and they do) they go really bad.
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Old 10-11-2015, 11:56 PM
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Very good. I think I'm up to machining a outboard seal housing........................I prefer that option.

And yes, it is a 17x40 bearing. I assume they both will be the same, but have not yet been able to set up a suitable press to remove the shaft and the other one from the other case half.

Last edited by Jim.Thompson; 10-12-2015 at 12:02 AM.
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Old 10-12-2015, 04:01 PM
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You best measure the rear bearing too? Stihl is notorious using a odd ball proprietary bearing on the clutch side. And the MS 660 was no exception.
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Old 10-12-2015, 04:20 PM
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Yes I plan to. As soon as I get it out. Still looking for a suitable round spacer to support the case while I press the shaft and bearing out.
I have to put it aside for a while and get some composite work done before it starts to rain big time. I have a couple of moulded models to pull out of the moulds for flying buddies. I'll have lots of time to concentrate on it when the wet starts.
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Old 10-12-2015, 05:09 PM
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There's a special tool for getting the case halves apart. The tool looks like a big C-clamp. The "C" part reaches inside the case and pushes (It pulls?) on the case towards the outside and the screw pushes against the end of the crank.

If you saw a picture of the tool, you could make one?

I have the Stihl 066, MS 660 workshop manual and illustrated parts if you need them but I will have to E-mail them to you?

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Old 10-13-2015, 07:21 PM
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Thanks w8ye. Now the case is stripped, it might require a different tool. I managed to press the shaft out of the flywheel end bearing, but only after I had cut away much of the surplus alloy case. My puller then could grip the remaining flange and not the gasket face. Both bearings are the same size, 17x40, so that makes things easy. Also, the seal is 17x23, which might have to be genuine. Unless I machine the bore out to take a larger OD.
I'll see if I can post some pics. Not sure how to use this system yet.
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Old 10-13-2015, 07:42 PM
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Originally Posted by ZAGNUT View Post
..................but only if you cut the unused part of the crank off. .......................
I do plan to cut the unused part of the crank off and blank off that end of the case. That formerly used for the clutch.
How do you suggest I do this? I don't want to press the big end bearing and pin out unless I absolutely have to. If I did, I would then have a problem chucking it in a lathe.
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Old 10-13-2015, 09:53 PM
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put the crank in a garbage bag with only the clutch end poking through a tight hole and use a thin cut off disc in an angle grinder. go slow and even cool it with water from a spray bottle to keep it from heating up. clean it up and chamfer with a grinding disc or even hand files and then carefully remove the bag so as not to contaminate the big end of the rod.

you could also put the old bearing back on to use as a cutting guide but it gets harder to seal off this way. no need to be pretty here, just get the length of the stub close to the width of a standard bearing.

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