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OS 120 Surpass I power loss

Old 09-06-2003, 12:44 AM
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Default OS 120 Surpass I power loss

I recently had to replace the bearings in my OS 120 Mk I series motor. After doing so it seems to run very smooth but I can't get the same high end as before. I used to get 9100RPM on Wildcat 15% with a 16x6 prop. The best I can get now is about 8800RPM with 20% and the same prop.

I am going to replace the piston ring as soon as I can get one but is there anything else I should look at. I read the OS FAQ on installing the timing cam but it looks like it could go in with the mark at the top or bottom and still work - mine is at the top at TDC. Should it be the other way? What happens if I am off one tooth either way?
Old 09-08-2003, 02:19 PM
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Default RE: OS 120 Surpass I power loss

You'll need to check how you have the cam gear set. It's not straight up or straight down when the piston's at TDC. It's aligned with the pushrod tubes.

See the FAQ:

http://www.osengines.com/faq/product-faq.html#q15
Old 09-09-2003, 07:56 PM
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Default RE: OS 120 Surpass I power loss

Bax;

I noticed in the flights since my post an improvement in power. I double checked the cam again and tried re-inserting again to be absolutely certain it is in line with the push rods at TDC and not perpendicular. I don't think I changed it any. In any case it continues to improve and after about a total of three full tanks (and a switch to 20%) I am back to 9200-9300 RPM with a 16/6 prop. I suspect it may just have been a breaking-in after the re-assembly. I am still going to replace the ring but I might save that for a winter project.

Thanks

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