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Have a little issue. Recently bored up .50 and have the .50 piston and rings. Problem is, rings won’t fit in the piston groove properly. I installed them properly and they won’t compress. Any suggestions. The rings have an RN designation on the top. Any clue on that? Thanks so much
Automotive tire Wood Gas Handwriting Font

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Can't quite tell there but if trying to install standard rings on a keystone grooved piston that will happen. Keystone rings are ones the top surface is angled rather than dead flat, many Japanese 2 stroke bikes switched to them in the late '60s-early '70s and you have to have the correct rings to correct piston.

What the pic shows is exactly what happens, the ring sticks about halfway into the groove. Look carefully in bright light at the ring end, you will see the ring is rectangular with right angles on all corners, keystone will have an angle on the top of ring, it is thickest on the outside and thinner toward the inside and ring groove in piston is cut the same way. You cannot interchange parts that are the old square type. A keystone ring WILL go into a square piston groove but the ring will be way down in compression due to the fact the ring cannot seal correctly in the groove. The keystone top angle was designed to increase ring sealing to add compression, the angle gives more area for the combustion gases to push the ring out harder to seal better. They also scrub the land to keep it clean of carbon to stop ring sticking.

Take note, sometimes one groove can be keystone and the other not.

The 'RN' is likely the ring maker.
 
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