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



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Joined: 1/1/2003
From: LEXINGTON, KY, USA
Status: offline 		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

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



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Status: offline 		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.

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



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

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



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Status: offline 		When you remove the head, the pushrod will fall out

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



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From: LEXINGTON, KY, USA
Status: offline 		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

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



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Status: offline 		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.

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



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From: Paris, ON, CANADA
Status: offline 		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.

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



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From: LEXINGTON, KY, USA
Status: offline 		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.

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



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Status: offline 		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


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



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Status: offline 		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.

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



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Status: offline 		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.

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



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Status: offline 		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.

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



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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

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


 

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Status: online 		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.

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



Posts: 333
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Status: offline 		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.

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