os 90 max 2 stroke compression - RCCanada - Canada Radio Controlled Hobby Forum
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Old 01-12-2008, 02:02 PM   #1
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os 90 max 2 stroke compression

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I have an os 90 max 2 stroke that is approximately 25 years old. The engine runs fine and seems to have lots of power to taxi a 1/5 scale waco. I would like to use this engine in a funtana 90. My concern is that when I turn the engine over by hand there seems to be low compression compared to an os 46 fx 2stoke. I've been told that the 90 is a ringed engine and the 46is an abc engine and that they will have a different feel when turning over. Any views on this subject would be appreciated.
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Old 01-12-2008, 03:13 PM   #2
Ed Smith

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What you were told is correct.

In simple terms a ringed engine relies on the fast build up of compression to expand the ring and make it seal. If turned over slowly the ring is loose in the groove and allows leakage. This is generally more noticeable on well used engines.

A tapered sleeve engine should not be turned over cold. The fit of the piston and sleeve is correct only at operating temperature. If the engine is turned over cold and feels tight at TDC then the piston is being forced into the taper possibly resulting in damage to the piston.

Ed S
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