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Jan 18, 2016, 08:59 AM
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Piston Ring for OS .50SX

Hello all, OK, I've managed to completely dis-assemble my OS .50SX and found that it needs new bearings. I also found that the piston ring appears to be frozen or gummed into the piston groove and it's currently being soaked in acetone to help loosen it up. I'll probably have to replace the ring and wanted to ask if anyone has a good source for a ring. The piston itself and the cylinder liner look to be in good shape and I was wondering if they should also be replaced. Also, is it worth putting a lot of $$ into this or would it be better to just sell it for parts. Once again, any and all help would be really appreciated. Thanks very much!! Jack
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Jan 18, 2016, 09:52 AM
Registered User
I would not replace anything until I have it loosened up and run it. Often the bearings are just a bit rough from sitting, and the oil drying up, as well as the rings getting stuck. Just soak for a bit in acetone and then flush with fuel before making a decision. If the bearings have damage like a broken cage or nasty rust, well then ya I would replace them, but would not get any parts unless they really need them. OS stuff is a lot of money.
Jan 18, 2016, 12:49 PM
Closed Account
Yup.... Agree with that completely! A stuck ring is just stuck, nothing else. When it gets stuck, it stops wearing. Soak the pistontop in acetone, and carefully work it loose.
After it came loose, soak it some more, periodically moving the ring about a bit, to work out the dirt behind it.
Then check how the anti rotation pin in the piston is, and locate the ring gap at the pin.
Assemble, and 99.9% chance, everything will be all OK again.
Jan 19, 2016, 07:55 AM
Registered User
Thread OP
OK...thank you. I'll give it a try and see if I can get that ring loosened up, it is really stuck and so far, after soaking it in acetone, hasn't began to move. I'll keep soaking it for a few more days and see if that works. I wish there was a solution I could put it into and then heat it on the stove without catching fire. Any ideas on that? Anyhow, the bearings appear to be OK after soaking them and getting all the goo out of them. I've never seen an engine this gummed up, don't know what the heck kind of fuel they were running or how long it was sitting. Thanks again, Jack
Jan 19, 2016, 09:11 AM
Registered User
If the ring is pinned, it may not want to move, especially if it is carboned up and stuck. I have had some pretty nasty motors from swap meets, but they all seem to go alright after a cleaning and going over. Maybe 3 out 150 were really scrap. Oh and I ruined two by playing with them. One was with a pinned ring.
Jan 19, 2016, 09:15 AM
Master re-kitter
SRQFlyer's Avatar

Stuck ring

Aim your covering heat gun right at the ring (on high) for a minute or two. That may loosen the gunk enough for an end of the ring to pop out - maybe with a little help (careful). Then the acetone can get behind the ring somewhat and do its job.
Good luck,
Jan 19, 2016, 09:39 AM
Registered User
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Sounds like some real good advice. I'll try some heat and see what happens. Then I'll soak it in acetone again and hope it moves. If not, I guess I'll put it all back together and try to run it and then see what happens. I don't want to mess with it too much and ruin it. I hope I don't have to sell it for parts, it's just too nice of a motor. Thanks very much for all the help, I really appreciate it! Jack
Jan 19, 2016, 10:08 AM
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Parts can still be found.
Jan 20, 2016, 08:15 AM
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Well it looks like the heat gun did it. I managed to lift both parts of the ring where the gap is, out of the piston carefully using the sharp tip of a #11 exacto blade, now I'm soaking it again in acetone and will heat again later on to try and free up the remainder of the ring. I'm not sure if I want to try and remove it from the piston to clean under it (in the piston ring groove) or not. It looks pretty fragile and I certainly don't want to break it $$$. Thanks for some great information. Jack
Jan 20, 2016, 09:55 AM
GloBroz PowerLab
1QwkSport2.5r's Avatar
If you have the ring free, you will want to remove it from the piston to remove the carbon and oil crud from the ring groove. If the groove is full of crud, there is less area for combustion pressure to get behind the ring to force it out against the cylinder wall leading to a loss of combustion pressure. It will certainly run without cleaning it, but it likely will be down on power. You don't want to have to take it apart again and have to get the ring to re-seat again. This video shows putting the ring back into the piston, but removing it is just the opposite. Lift one end of the ring out of the groove and move it to the top of the piston. Then just rotate the ring around the piston to remove it.
Piston ring install in model engine (1 min 39 sec)
Pardon the dork in the video talking.
Jan 20, 2016, 03:45 PM
Registered User
Thread OP

Stuck ring now broken ring

Darn, I thought I had it made. The ring was loose except for one spot opposite the gap. I heated it again and was working it around to remove it and suddenly (you guessed it) snap and it was broken. I still had a heck of a time getting that last little piece out, man was it stuck in there. So if anyone knows of a ring available for an OS .50SX please let me know and can I still use the same cylinder liner? it appears to be OK. Wow that was frustrating!
Jan 20, 2016, 06:49 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
eBay: OS original factory ring item number 251612592856
Reproduction ring item number: 291498918397
Jan 20, 2016, 10:07 PM
Registered User
Bone's Avatar
UK based seller manufacturer of rings. I have purchased rings from this fellow, fitted perfectly in my OS Max-H 40.
Jan 21, 2016, 07:59 AM
Registered User
Thread OP
I surely appreciate this great information. Now I can order the ring and get my engine back together. Thanks very much!! Jack
Jan 21, 2016, 08:21 AM
Ron - AMA 1025
ronwc's Avatar


I have a new ring I will sell - also will throw in used piston for free - Ron

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