Os 120 4 stroke overhaul help needed (Full Version)

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Cub Man -> Os 120 4 stroke overhaul help needed (11/11/2007 4:18:00 AM)

Hey guys just picked up a fs series os 120 4 stroke. It is an old one most that I have seen have one valve cover for both valves. This one is a cove for each valve. Any way this engine has been setting for years. I need to overhaul it. I started tearing it down. I removed the set screws that hold the pins that the rocker arms are on. Now how do I get the pins out? Any help is great as this is my first 4 stroke. I have been in the hobby 14 years and just now getting a 4 stroke What is the best stuff to use to clean an engine up



w8ye -> RE: Os 120 4 stroke overhaul help needed (11/11/2007 4:36:40 AM)

I wouldn't remove these pins unless something is broken about them?

You cannot get new parts for this engine from the OS sources but the bearings and ring are commonly alailable. That's generally all you will need.



Cub Man -> RE: Os 120 4 stroke overhaul help needed (11/11/2007 1:31:28 PM)

one of the push rods coming up that works the valve is bent



w8ye -> RE: Os 120 4 stroke overhaul help needed (11/11/2007 2:25:11 PM)

When you remove the head, the pushrod will fall out



Cub Man -> RE: Os 120 4 stroke overhaul help needed (11/11/2007 2:28:30 PM)

It appears to be two allen bolts under the rocker arms that hold the head on. I dont think i can pull the screws with the rockers in place



w8ye -> RE: Os 120 4 stroke overhaul help needed (11/11/2007 2:37:47 PM)

I'm sorry, I have never owned a pre -Surpass double rocker cover OS 120. I've ran, adjusted, and flown them but never had to disassemble one. My overhaul experience has been with the newer engines.

I do know how to set the cam timing and adjust the valves.



Waco Driver -> RE: Os 120 4 stroke overhaul help needed (11/12/2007 4:09:26 PM)

Tap the rocker arm shafts out using a 1/8' pin punch or a piece of music wire using care not to damage the nylon washer on the inside face of each rocker arm. Tap the shafts inboard towards the center of the cylinder until the rocker arms can be removed, then push the shafts outboard to remove. They are probably stuck with congealed oil.

Soaking the engine in some fuel or methanol overnite will probably loosen things up.



Cub Man -> RE: Os 120 4 stroke overhaul help needed (11/12/2007 4:13:25 PM)

Thanks for the help. First I will say guys that I will probably never sale the engine as it was a on a plane that belong to a flying friend that has passed away. I’m looking to get it back up and running so that this plane will see flying time again.



Ed Cregger -> RE: Os 120 4 stroke overhaul help needed (11/12/2007 5:19:00 PM)

Cook it in antifreeze and do not disassemble it any further than absolutely necessary. As Jim said, parts are no longer available from the US OS importer and can be very difficult to find.

This engine has gained one heck of a following since it was dropped from production. It is a sweet running engine when right, but there are a few caveats. Pay particular attention to the high speed mixture. Never - ever - let it get even close to running at absolute peak rpm. The crankcase has been known to break in two if the engine backfires. Keep it fat beyond reasonable doubt.

The pushrod should be able to be straightened with a soft mallet and a hard wood, or steel, surface. Just take your time and be sure it doesn't fly off somewhere never to be found again.

I bench ran my FS-120 (official designation) only since it was one of the first imported into the country and I didn't have a model of the proper size for it. It ran fine on the bench, lots and lots and lots of times before I sold it to a friend, who sold it to a friend. The friend's friend had a model, mounted it up and flew it one summer. Before the summer was out, he had to replace the rocker shafts and associated parts due to a lack of lubrication. Another caveat.

Good luck getting her going.


Ed Cregger



w8ye -> RE: Os 120 4 stroke overhaul help needed (11/12/2007 5:26:50 PM)

In the field where I flew in Alabama, A well known modeler was gently flying his Monocoupe with a double rocker 120 when "BANG" the bottom of the crankcase blew off.



blw -> RE: Os 120 4 stroke overhaul help needed (11/13/2007 12:15:54 AM)

Yes, it did go bang right over my head!

It had a couple of microseconds of the engine reving up and it went. It did blow out the whole bottom of the crankcase. I think it was in two pieces in the bottom of the cowl. The guy had mentioned that he thought it was running hotter than normal, but he flew it again that day.



donkey doctor -> RE: Os 120 4 stroke overhaul help needed (11/15/2007 3:17:17 AM)

Hello; I am in the process of overhauling an OS FS120 right now, there are a few things that no other engine has; The rockers have to come off to expose the rocker carrier and allow access to the head bolts. There are plastic shims on either side of the rockers that are tough to get back in. You can find parts, if you need them, no exhaust pipes though. Noel Jensen has parts.
Upon re-assembly I found that the new push rods were longer then original, now I have to find shorter adjusters somewhere, or make them.
The engine I am overhauling had excessive cam wear and needed a cam and lifters and pushrods, due to a lubrication problem. I think the woner tried to lean it out for more power then it can make, then the damage happened.



jessiej -> RE: Os 120 4 stroke overhaul help needed (11/15/2007 4:58:42 AM)


quote:

ORIGINAL: w8ye

In the field where I flew in Alabama, A well known modeler was gently flying his Monocoupe with a double rocker 120 when "BANG" the bottom of the crankcase blew off.


I bench ran my old double rocker 120 a couple of weeks ago. It is a very smooth runner, but I had been unaware of the potentially explosive nature of these handsome old engines! Thanks to these posts I will be extra careful about lean runs and lubrication.

The example I have was purchased new. Aside from bench running I had it in a Lazy Ace for a few flights then replaced it with an OS 1.06FSR. I think it is a more attractive engine than the current crop, but that is purely subjective.

jess



submikester -> RE: Os 120 4 stroke overhaul help needed (11/15/2007 6:18:33 AM)

I've got one of these and it's a pretty sweet little engine. Never a problem and I always run it a bit rich.



Cub Man -> RE: Os 120 4 stroke overhaul help needed (11/19/2007 12:51:59 AM)

Good news. The rod was not bent it was just popped out of socket. I will give the engine a good cleaning and put it back together. Everything looks to be in great shape.



Jaybird -> RE: Os 120 4 stroke overhaul help needed (2/12/2008 10:28:50 PM)

Does anyone have the factory recommended settings for the high and low speed needle valves for this engine?

I've sent an e-mail to a service address in another thread but I haven't heard back.

Jay



Kimhoff -> RE: Os 120 4 stroke overhaul help needed (2/13/2008 2:18:46 AM)

Same as a 91 Surpass I



Jaybird -> RE: Os 120 4 stroke overhaul help needed (2/28/2008 7:03:21 PM)

Does any know the thread size for the pushrod adjustment nut and set screw? The set screws on my very used engine are broken off at the top hex area (I have no idea why). There is a lot of slop in the rocker to valve clearance, although the engine will run. I hate to start taking it apart and risk further damage without knowing if I can replace the set screws (at least I think they are cup point set screws, like Saito uses) with standard hardware. The reason I ask is because the modern O.S. manual shows what looks like a socket head cap screw inverted on the pushrod end of the rocker arm with a jam nut on top. Not sure I can replace that kind of part.

Thanks for your help!!!!

Jay



donkey doctor -> RE: Os 120 4 stroke overhaul help needed (2/28/2008 9:30:56 PM)

Hello; Noel Jensen has your valve parts. "The rocker adjuster jam nut sits just below the top ridge of the rocker box. so you need a wrench with a little bend in the neck to get the handle clear of the rocker box. I think it's 5.5 mm.

Another thing I noticed but didn't mention while doing the overhaul of the os fs 120 was to look at the wrist pin. If there is any suggestion of a lack of lubrication, the wrist pin will likely be damaged. The wrist pin in my engine was toast, and would have failed had I re-installed it.

That engine is generally over engineered, the connecting rod is huge for the forces it has to deal with, but don't try to run it lean or squeeze every last rev out of it. The lubrication system is marginal.



Jaybird -> RE: Os 120 4 stroke overhaul help needed (2/29/2008 3:12:54 PM)

Thanks Doc. Is there contact information for Noel Jensen? I didn't see it in the thread although he's refered to many times.

Thanks,

Jay



donkey doctor -> RE: Os 120 4 stroke overhaul help needed (3/2/2008 4:04:57 AM)

Hello Jay; I bought a new computer and don't have Noel's e'dress in the address book in the new computer, so I guess I have lost him.



w8ye -> RE: Os 120 4 stroke overhaul help needed (3/2/2008 4:24:49 AM)

Noel Jensen
204 S Division St,
Whitehall, MT 59759
406 287 3270
nubs@mybluelight.com



Jaybird -> RE: Os 120 4 stroke overhaul help needed (3/4/2008 3:49:06 PM)

Thanks a lot for the address. I've written him and he responded a few minutes later. I'll have parts on order soon.

Jay



GuyIncognito -> RE: Os 120 4 stroke overhaul help needed (3/6/2008 7:45:08 AM)

Jaybird

This is a little off topic but what make are the floats you used on your E-Flite Taylorcraft and where did you get them. You said they are off of a Sig EP42 but Sig only sells a wood float kit that size.

Thanks, Bill T.



Jaybird -> RE: Os 120 4 stroke overhaul help needed (3/6/2008 10:32:47 PM)

Hi Bill,

Sorry, what I meant to say was I HAD them on a SIG Kadet EP42. I bought them through FoamFloat.com who unfortunately was out of business last time I checked.

Jay



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