OS GT55 Gas engine issue - RC Groups
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Feb 18, 2011, 07:10 PM
Registered User

OS GT55 Gas engine issue

Hey group,
We have two of these on 50 CC extras.
My son was flying and when he landed heard a pop and realized the prop was spinning freely.
Turns out only thing holding the wooden prop on was the spinner.
Must have happened close to the end of the flight.

We busted all four bolts and are having a hard time getting them out of the prop hub.
I drilled a small hole and used a knife thinking they would be loose but wont budge.
How do you get the prop hub off?? anyone know?
I see that its bolted from the front pilot shaft but looks like you need a special tool to get to it inside the hub.
Help would be appreciated.
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Feb 18, 2011, 11:12 PM
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earlwb's Avatar
wow I am impressed, I didn't think one could shear off all four of those bolts at one time. That is something new to me.

In looking at the instruction sheet that comes with the engine, and the review of the engine, where they disassembled it here http://downloads.hobbico.com/reviews/osmg1555-rcsf.pdf
I would say that the drive spacer is a very light press fit onto the crankshaft. They have what looks like a drive key near the rear maybe a screw like pin. Then there is a woodruff key on a slot on the inside, next there is a tapered collet at the rear to help keep the drive spacer centered.

So anyway, it looks like the drive key pin is unscrewed, next unscrew and remove the ignition timing sensor, so it isn't damaged by the heat or something, and then you heat up the drive spacer some, then holding it carefully in a leather gloved hand, gingerly tap on the crankshaft tip, or the pilot shaft tip, with a brass hammer, and the drive spacer should pop loose. You want to have something like your lap or someone else ready to catch the engine when the drive spacer comes loose.
what I cannot tell is if you unbolt the pilot shaft from the crankshaft before or after you get the drive spacer loose. sometimes you have to hit them harder, but always start off easy and work your way up to more force. What likely has the drive spacer stuck is the tapered collet at the rear of the spacer.

Using a brass hammer minimizes the shock effect that might cause a hardened steel shaft or hardened steel bearing races to crack or break off. Sometimes a steel hammer will cause the hardened steel bearing races to crack or a crankshaft to break apart.

I have had to occasionally break out my antique BFBH before on recalcitrant motorcycle bearings that didn't want to come out easily. But I have several different weight brass hammers I can use situations like this. Unless a engine or wheel bearing begins to get me upset, I seldom need to resort to this sucker.
Last edited by earlwb; Feb 18, 2011 at 11:27 PM.
Feb 19, 2011, 12:07 AM
Registered User

pilot shaft

I thought the pilot shaft was holding the prop hub on??
so if i can get a correct elongated socket and took the pilot shaft off it would be able to slide off??
Feb 19, 2011, 04:00 PM
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earlwb's Avatar
I reviewed it and yes you are correct in that you'll need a special super extra deep socket to fit the pilot shaft nut. Sorry my mistake.
I doubt OS will sell it to anyone either. I think you could make one by welding another socket onto the one that fits. If you can find a socket that has the hex head on it to fit a box end wrench, that might be a better way to go.
The extra deep sockets I saw on the internet didn't look quite long enough, but maybe I missed some.
Feb 19, 2011, 05:42 PM
Registered User

Os gt55

Kind of a crappy way to sell an engine..
We were running a wood prop and sure that it got just a little loose..
They should supply the tool if they are going to sell something that takes special tools to fix ..
I have two of these engines and will make sure the other plane prop stays very tight.
I am in touch with them and will see what they say,
Feb 19, 2011, 09:37 PM
Registered User
earlwb's Avatar
Well, I can sort of agree with you too, but I do not know of any car company or motorcycle company that sold any of their special tools to the public either. I had to look long and hard to find some special tools for my Harley. So it is about par for the course. I think they consider the engine a non-user serviceable item just as many other companies do. One is supposed to send it in for repair and pay them for it. So they aren't obligated to make it easier for someone to DIY it. Now considering how much those extra deep sockets cost as I don't see them being sold individually (unless Snap On or Mac tools does off their truck dealers) so it would be expensive to have to buy a set. So maybe there is a small market for someone to fabricate a socket tool for this purpose and sell it.
Feb 19, 2011, 10:59 PM
Registered User


I suppose that we should be counting our blessings that the prop did not fall apart with the stress against the spinner while in a 3D situation.. that would have been no fun..
Feb 20, 2011, 08:20 AM
Registered User
earlwb's Avatar
Yeah I think so.
I am still back there thinking it might be a defect with the bolts being overly case hardened and made brittle. I would have likely sent the engine back to see if they fixed it under warranty or not.
Feb 20, 2011, 01:39 PM
Gas Only
Fokker DII's Avatar
Originally Posted by earlwb
Yeah I think so.
I am still back there thinking it might be a defect with the bolts being overly case hardened and made brittle. I would have likely sent the engine back to see if they fixed it under warranty or not.
There was a major recall of hardware where I worked a few years ago. Seems that some of Chineese hardware was fraudelently marked as to material type and was failing in high stress aplicatins due to (as I recall) hydrogen imbrittlement. Could be more of same??

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