OS 40SF - compression

I picked up a OS Max SF40, ringed version, at a swap meet some while ago. Whilst I knew that it had poor compression and probably needed a new ring I have only just noticed that the compression is quite good if you flip the prop backwards. Flipping it in the normal direction it is quite soft and gives some hissing at the exhaust port.
How do you explain this?

The ring is installed upside down, perhaps. Or maybe it has a LH crank. [photo not recovered: sipcof.gif] Steve

Steve Helmick
Renton, Washington
USA

The ring gap has worked its way to the bypass side of the engine. It is most likely worn also. This happens on engines with unpinned rings.

Send it to Frank Bowman and let him blueprint the engine. It'll run ( probably) better than new.

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>Send it to Frank Bowman and let him blueprint the engine.<
>It'll run ( probably) better than new.<
>[email protected]<

I have one of these with the RC carb on a RC ship with a Frank bowman ring in it. It's a runner as I guess most of them are. Break in has taken much longer than a new stock engine. But it really runs good.


Greg L. Bahrman

I'd be interested to learn more on this. It's a well known phenomenon, and has sometimes been attributed to wear caused by increased side pressure on the power stroke. It is seen on motors with two rings where the gaps are on opposite sides, so the gap may not be the only cause. Back in the late 50's/early 60's some of our speed fliers claimed to rejuvenate their Dooling 29's by reversing the c/case when this condition developed. Anyone got any validated explanation for the condition?