Engine Decarboning: Piston 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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The cylinder heads, cylinders, and piston rings should have been removed by this point. Mark each piston for reassembly in the same position. To remove the pistons you'll want to lay the engine on it's side. When the wrist pin is removed from the piston, the 31 tiny pieces of hardened steel and the two steel rings that make up the wrist pin needle bearing will all fall out of the connecting rod end. You really, really, really don't want these things falling into the crankcase or onto your shop floor. Ask me how I know.... Anyhow, put blocks underneath the case so that the weight of the engine is taken by the fan tower and the crankcase not by the cylinder bolts. Drape the bolts with clean rags. It's kind of like preparing your patient for surgery....
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First step is to remove one of the two spring steel circlips which retain the wrist pin in the piston. Wear safety glasses. You only want to take out the one in the PTO side of each piston. Being careful not to scratch or gouge the surface of the piston, insert a thin screwdriver or scratch awl into the slot behind the pin and pry out to start the clip out of its groove. Get ahold of the clip as soon as you can with forceps or needlenose pliers or cover the whole operation with a cloth or rag to catch it. You're not going to keep or reuse it, but you could put your eye out with that thing when it pops out of there at a bazillion miles an hour. |
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If there isn't a lot of varnish built up in the wrist pin bore of the piston, you may be able to reach around back to the mag end of the wrist pin and push it out toward the PTO end with your finger. If it won't move or starts out then gets tight, try warming up the piston by applying heat to the piston crown with a heat gun while continuing to apply pressure to the wrist pin. The magic moment occurs when the piston heats and expands before the pin does; once the pin heats up and expands it'll bind up again. If it still won't come out, go to plan 'C' - next photo.
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Plan 'C' is to press out the wrist pin by putting a socket small enough that it will fit inside the circlip against the mag end of the wrist pin - 7/16" worked for me - just visible at 'A'. On the PTO side, place a wood block against the piston that has a hole in it which is slightly larger than the diameter of the wrist pin. Put a clamp across the whole rig and squeeze the wrist pin out. Because of the limited room between the fan tower and the piston on the mag end piston you may need to start with a short socket then remove it and insert a deep socket in order to press the wrist pin far enough through. You only need to get the wrist pin past the rod end bearing to allow the piston to be removed from the rod. As before, a little heat applied to the crown of the piston will help move things along. I like the squeeze clamps for this job because they make it difficult to apply damaging pressure if you don't have something lined up just right. |
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Once the wrist pin is past the rod end bearing, the two thrust washers and most of the 31 bearing needles will fall out. With the engine upside down or on it's side, all this stuff will fall into and be retained by the piston. This is a very good thing.
Remove the piston and inspect the rod's bearing surface and all the needles for any pitting or corrosion (if any is found, you should really tear the engine down completely; there's likely to be wear and/or damage elsewhere) and bagged. Label the bag to identify the piston it belongs to. Then if it's not already out, finish removing the wrist pin. A cautionary tale: When I did the first one, I had the engine upright figuring that the bearing parts would stay in place around the socket. Wrong. They fell out and some made it past the rags I had covered the crankcase with. When I calmed down and retrieved all I could see in the crankcase with a magnet I was still two short. You absolutely cannot leave anything in the crankcase that doesn't belong there. I finally spotted the very end of one of the little critters peeking out of the lube hole that leads to the PTO crank bearing. What were the odds of it falling in there? The magnet fished it out but I was still one short. I finally spread out a clean pillow case and turned the engine upside down over it. I don't know where it was, but to my great relief, number 31 dropped out onto the cloth. Learn from my stupidity; turn your engine on it's side.
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