Engine Decarboning: Piston Ring Removal
WARNING: The author of this page is not a professional or certified aircraft engine mechanic. The information contained herein constitutes a description of the writer's experience only and should not be construed or interpreted by the reader as instructions or guidance for the successful completion of any repair or maintenance procedure. Use of any information contained herein is at the user's own risk. This information has not been reviewed, evaluated, or approved by the manufacturer of this engine.
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Even though these photos show the pistons already removed from the engine, I recommend removing the rings from the pistons before taking the pistons off the rods. This will minimize the possibility of damage to the rings from getting knocked against the cylinder bolts during circlip, wristpin, and piston removal. The top ring is removed first. Rings must not be stuck tight at any point before removal. Stuck spots can be worked loose by soaking the ring with kerosene followed by gentle pushing and wiggling starting from the ring ends and working around to the sticking point. If necessary, remove the piston from the engine with the rings in place (next page) then immerse the piston in kerosene overnight or until the ring can be worked free. The ring may be binding but it must be able to move at least a little as you proceed or you'll risk snapping it at the point where it's stuck. The top ring is especially pricey so be careful. Once you've got movement all the way around, gently remove the ring by pushing on one ring end while the other end bears on the locating pin inside the groove. On the top ring this pin is located in line with the intake manifold on the carb side of the engine 'A'. With a lot of carbon in the groove the pins can be hard to see but they're there. Push out just enough to be able to start to lift the ring toward the top of the piston then work your way around gently working the ring up and out of it's groove till it comes free. Don't pry upward or outward excessively; rings are brittle and can snap.
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The bottom ring can be removed in a similar way. Its pin 'B' is located about 30 degrees toward the PTO end of the engine from the top ring pin 'A'. Alternately, you can use a ring expanding tool to spread both ring ends until you can work the ring out of it's groove on the opposite side. Go easy; it's easy to spread the ring too much and either snap it, deform it, or cause the molybdenum coating to seperate from the ring. Spread them gently and only enough to work them up and off. |
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As soon as you remove the bottom ring, and keeping it with the same face up that it had on the piston, wipe about the first inch back from the ends of the rings clean (there shouldn't be much carbon on these surfaces). Then examine the ring with a magnifying glass. Just back from the ring ends are some tiny little letters made up of little dots. They're on both sides of the gap but are only on one side of the ring. On my engine they were on the top side of the ring - the side toward the piston crown. Note which way is up for your rings so you can put them back the same way on reassembly. Can't see em? Just clean the oil off with solvent and make your own mark with a permanent marker. Important: Make sure you keep track of which rings go on which pistons. I keep the whole piston assembly including wrist pin, bearings, rings, and piston in a zip-lock bag to make sure they all stay together.
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