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Gas Porting the "early" YS Four Strokes
Author: Mark Fuess

Early YS four Strokes used "conventional" piston and ring configuration which works good while the engine is new, or in  relatively good condition. The problem developes when the piston ring starts getting worn and blow-by begins to effect the crankcase pressure. This causes all kinds of tuning problems! To solve the problem, the piston is Gas Ported so the compression will push the piston ring against the cylinder thus preventing (or minimizing) blow-by.The new engines are already set up this way, but the older engines need this modification. Here is how you do it...

Completely disassemble the engine. Thoroughly clean all the parts and inspect every part for wear or damage. Remove the piston rod and ring from the piston. The modification requires drilling 3 holes on the top of the piston towards the edge of the piston. The holes need to be positioned so gas pressure from the cylinder will be behind the piston ring pushing the ring against the cylinder.

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Look at the illustration and get familiar with orientation of the holes. You will notice the the drilled holes DO NOT go below the lower ring groove. Once you have drilled your holes, use your old piston ring to clean the ring groove. Do not install your old piston ring. 

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This is my piston being ported on an endmill and rotary table. This is totally un-necessary but it sure does a nice job.

On the new YS engines these "holes" are actually slots. It's just a faster way to get the job done.

Once you have completed the porting clean the piston and re-assemble the engine using a NEW piston ring, gaskets, and O-rings.

Refer to my link for engine re-assembly: http://tech.flygsw.org/ys91.htm  This will walk you through the entire process step by step.

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mark.fuess@comcast.net