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OS FS-120 Disassembly

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OS FS-120 Disassembly

Old 06-19-2015, 09:35 AM
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Small Block Mike
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Default OS FS-120 Disassembly

Now that I've finally retired, I'm getting back into RC after a 25 year hiatus. I've uncrated all my old planes, both gliders and powered, and have started installing new radios. I have an old OS FS-120 four stroke that I purchased in 1986 (see image) that unfortunately had not been properly pickled for storage. Consequently, I found it pretty gummed up and would not turn over. I got it partially disassembled with the liner out, but need to get the piston and crank out. The liner looks great with no signs of rust or wear. The engine only had about 1-2 hours on it. I can not find any instructions for removing the piston and crank. Can anyone help? I suspect I will need to replace the bearings.

Any suggestions and help will be appreciated, particularly the best place to get parts such as the bearing, and proper disassembly instructions.

Thanks,
Michael, AMA 8970
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Old 06-19-2015, 11:40 AM
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Gray Beard
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Heat is your friend. You will have to heat the engine up then take it apart and clean all the old castor gunk off the parts. Some people just stick them in the oven at about 250 until it loosens up enough to roll over. I use a pot of boiling water and just drop the whole engine in the water. You first Remove the top end and all related parts, the cam, lifters push rods. mark the top of the piston so it goes back in the same direction. Your engine should have a hole in the back that you use to line up the gudgeon pin that is holding the piston. You can use a round tooth pick to pull this out through the hole. Once out the piston is free then the rod.
Go on line and try to locate a little book called A Guide to OS Model 4/Strokes by Graham C. Rice. It was sold through RCM in the 90s. I had a problem with OS service back then and I bought this little book that ended up saving me a ton of money by doing my own engine work. I had built a lot of car engines but these little engines were something I knew nothing about at that time. It's well worth the time to locate.
Bearings I get from Boca or rcbearings.com or you can get OEM from Tower. Lot of choices, rings from Frank Bowman, he has a web site.
Old 06-19-2015, 11:55 AM
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I had a 90 and I found lots of youtube videos to help with. There is guy on ebay that sells lots of OS parts goes by the name of nubbdoesparts He has rings bearings etc. Very helpful guy and will walk you though all the process. Welcome back.
Old 06-19-2015, 01:44 PM
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Small Block Mike
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Thanks for the suggestions. I was able to get the engine basically disassembled to the point where I had the back plate, rocker housing, pushrods, and cylinder liner. I couldn't get the gudgeon pin out, but maybe with your suggestion I'll be able to do so. I removed the cover fasteners that hold the cam in place and it will rotate but not come out. I will come out approximately 1-2 mm, but then feels like it is being held by something. At this point, I went to Good Will and spent $3 on an old crock pot and tried heating everything in both automatic trans fluid and automotive antifreeze. This cleaned up a lot of the gunk, bit the cover over the end of the cam still will not come out.I also can't get the lifters out and didn't know if the cam has to come out first.
Old 06-19-2015, 01:47 PM
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Small Block Mike
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Thanks, I'll give the YouTube videos a try. I'll also check the ebay guy.
Old 06-19-2015, 03:09 PM
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Gray Beard
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If the lifters aren't out first the cam tends to get stuck on them. You need to tap the tooth pick into the pin a little so it seats well then rotate the pin and it should just slide right out through the hole. If the pin doesn't rotate it's still stuck in the piston with castor gunk.
Sometimes a shot of WD 40 helps too.
Once the rocker arm covers/tubes are removed the lifters should be easy to remove. A small piece of bent wire works well, just push the lifter from the inside of the cam housing a little and it comes out far enough to grab.
Old 06-19-2015, 06:33 PM
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Small Block Mike
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I think both the lifters and piston pin are stuck. I've shot them with WD40 and will check again tomorrow.

Question, if I decide to change the engine, what else would have approximately the same power and mounting hole dimensions so I don't also have to change the mount?
Old 06-20-2015, 07:26 AM
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I-fly-any-and-all
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Hello,
just thought I would ofer some friendly advice, ive been in the hobby since I was 10 and now im 26, when ever ive had a gummed up engine, and propane blow torch and some fresh oil do wonders!
about a year ago I bought 2 1950's ohlsson .19 ignition engines and about 10 cox engines came with the lot, all I did was heat and oil the engines and all run wonderfully.
However, there are still times when rust deposits can form.
I work in a machine shop and I prefer PB blaster penetrating oil or wd-40.
Old 06-20-2015, 08:38 AM
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Gray Beard
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Without sitting there with you I can't really tell what the problem is but remember, heat is your friend. Warning, heat can also warp the case. That's why we use the oven or boiling water instead of a torch. I have a small torch I use for soldering on my bench that I often use to spot heat an area but I use my covering heat gun a lot more so I don't harm the case. It doesn't require a lot of heat, just enough to expand the aluminum case so the steel parts will slide out.
Any 1.20 will fit back onto the mount. I use YS engines a lot more then OS these days and the 1.10, 1.15 and 1.20 would also fit the mount you have. So will the Saito but I would just do the repair on your OS. your just having a minor problem and it isn't anything new, you just have to do it once to figure it out.
You also heat the case to remove and install the new bearings.
Old 06-20-2015, 10:12 AM
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Small Block Mike
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Thanks for all the suggestions, I'll keep at it.
Old 06-20-2015, 10:51 AM
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I have 5 of those engines. Putting one back in the air this weekend. Purchased it 1984, added C&h ignition in 1989. Still works. Many bearing changes. A monocoat heat gun is hot enough to expand the aluminum case. OS used hollow steel piston pins with nylon caps on the end with a small hole. That is why the tooth pick suggestion. I also have one with aluminum plugs in the piston pin, also with a small hole. If all else fails use a small sheet metal screw instead of the tooth pick. Just thread it in with your fingers, no tools. Just enough so it bites and you can pull. Use the heat gun to expand the aluminum piston. Worked for me. Any lube will help. I use Marvel Mystery oil because it's smells nice.
Old 06-20-2015, 11:57 AM
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OK, made me go out to the shop and take apart an old 120 I have under the bench, it's a parts engine, screws only. It was running but the pump was worn out and you can't get the parts for it so it was run then put under the bench 16 years ago. I took photos of the removal of the lifters and gudgeon pin, that is a wrist pin to most of us auto guys and we called those little Teflon keepers Teflon buttons but what's in a name.
I will just walk you through it and you can see it all done. This engine was really stuck like it didn't want to roll over and nothing wanted to come apart.
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Old 06-20-2015, 12:54 PM
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It took me almost an hour and when the shop hit 100 I turned on the air. The lifters were really stuck but I heated the heck out of the case then used my high dollar tool {{Bent wire}} and pushed the first lifter out, it took a healthy push by the way! Removed the cam then removed the other lifter.
Then came the hard part. The wrist pin/ gudgeon was really really stuck. I heated the case then tapped in the tooth pick, the teflon button came out but the wrist pin was welded in. I got a piece of dowel then tapered it, tapped it in and still nothing. I finally used Marvel Mystery Oil and heat, still nothing!!. I used my pin torch and heated the piston and rod, got the pin to almost come out but the dowel would just pull out and leave the rod behind. OK, I got a bigger dowel so I could really jam it into the pin. Torched the rod and piston more and finally got the pin and piston to come out.
This took about an hour and a bit of thinking plus I didn't give up. I knew it all came apart so I just had to keep at it. Not going to let a machine defeat me. I have seen wrist pins so rusted in that the con rod had to be cut to get the engine apart, you do what you gota do!!
If I can get this old frozen up dog apart there is no reason to give up on a much better engine.
Mike, if you decide you can't get it apart and repaired give it to me. I always except free engines if it's an OS or YS!!! I refuse to rebuild Magnums and I'm not a big fan of Saito but I do except Saitos too.
Gene
Old 06-20-2015, 06:21 PM
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Small Block Mike
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Thanks Warbird20, I had concluded there were plastic plugs on the ends of the piston pin and used a small screw to pull it out. Still can't get the pin out so I soaked in WD20 some more and will use my heat gun on the piston tomorrow.
Old 06-20-2015, 06:30 PM
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Small Block Mike
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Thanks Gray Beard. I'm going slow as I don't want to force something when I do not know the proper disassembly procedure. Once I get the piston out, the next task will be the lifters. They are stuck and may be the cause of the cam cover only coming out 1-2 mm. When I rotate the crank, I can feel a slight interference every 720 degrees. That indicates to me one of the cam lobes is hitting a stuck lifter.
Old 06-20-2015, 07:01 PM
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Originally Posted by Small Block Mike View Post
Thanks Gray Beard. I'm going slow as I don't want to force something when I do not know the proper disassembly procedure. Once I get the piston out, the next task will be the lifters. They are stuck and may be the cause of the cam cover only coming out 1-2 mm. When I rotate the crank, I can feel a slight interference every 720 degrees. That indicates to me one of the cam lobes is hitting a stuck lifter.
I had to apply heat to the case to remove the cam cover and it still needed to be gently pried off. Once you get the first lifter out of the way the cam can be removed. The teflon button should stay in the wrist pin and it draws out the pin if it is freed up. In my case it didn't because the wrist pin was really stuck in the rod and piston, that is why I went to the dowels, the first dowel I used was too small so I went bigger and really wedged it into the pin. I used the small torch and really cooked the rod and the piston with pin point heat to loosen the wrist pin. As soon as I pulled it out the piston fell right off. Of course they tend to do that when they aren't attached to anything.
Old 06-21-2015, 03:38 AM
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Use a screw on the hollow piston wrist pin also if needed. It's on the inside and not the surface. You will have to clean the surface anyway, carbon or rust or both. Heat the piston again, preferably with the heat gun.
Old 06-21-2015, 08:43 AM
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Originally Posted by warbird20 View Post
Use a screw on the hollow piston wrist pin also if needed. It's on the inside and not the surface. You will have to clean the surface anyway, carbon or rust or both. Heat the piston again, preferably with the heat gun.
I'll let everyone know what happens later today.
Old 06-21-2015, 04:59 PM
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Well after hours of boiling it and soaking in WD40, using a heat gun, etc., I finally got the cam and lifters out. had to punch the lifters down after removing he cam. I still can't get the piston pin out after repeated attempts. I have to go visit a friend this week so I'll let it soak till I get back and try again. So far, nothing looks rusted, just terribly bound up with dried oil residue. The crank now spins very freely with no rough spots as it is rotated. The piston pin is so hard, I can't get anything to bite into it. Even tried a tap in the hole in the center of the pin.
Old 06-21-2015, 07:06 PM
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Look at the photo of the wrist pin, second from the last, that isn't rust, that's just old dried up Castor oil. I had to use the torch in the last photo several times on the piston to get it to loosen up. I came in through the back plate with the torch and fired up both sides of the piston that the pin is located through to get it to come loose. I heated it until I could spin it with the dowel, used some Mervel Mystery oil on it in those areas and heated and spun it more until it finally came out. That dowel is tapped in and really firmly seated to the pin. It's amazing how strongly old Castor will hold things. The heat melts it but it took a big bunch of heat to get it out.
The pin is hardened steel so I don't see anyone just using a screw in it. I never tried to tap one but I would be afraid the tap would break before threads were cut? As a last resort I have seen people use a small cut off tool and cut the connecting rod in half to get a piston out. Drastic and I have never needed to but if all else fails??
Old 06-22-2015, 07:55 AM
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We use an "easy out" screw extractor to remove stubborn wrist pins in the FS-120 Surpass engines. It's of hardened steel and works very well. A regular wood screw just doesn't have sufficient hardness to do the job very well.
Old 06-22-2015, 05:29 PM
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There you have it, if that's what BAX uses then give it a try. Reason I don't use the drill bit of easy out is I have broken too many of them but that was using them for extraction or forcing them but a wrist pin should not be a problem. Thanks for the tip Bill!!
Old 06-24-2015, 01:19 AM
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I've just replaced the bearings in my 120 Surpass 3.

The piston pin removal was the most difficult part. I used a long self tapping screw that was just SLIGHTLY
larger than the hole in the piston pin. The pin is hardened but the screw was soft enough to mash out when
I screwed it in. I then used the cutting jaws on pliers to lever the pin out.

John.

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