Engine Decarboning: Reassembling the Cylinders, part 2
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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With the intake and exhaust ports oriented to the correct side of the engine, lower the first cylinder onto the bolts and carefully guide it over the crown of the piston. Check to be sure the top ring gap is aligned with the pin in the top ring groove. With the bottom of the cylinder flanges contacting the top ring, pinch the ring from opposite sides of the piston pushing the ring into its groove. Work your fingers around the circumference of the ring pinching as you go until the cylinder slides down over the ring. Be careful not to let the cylinder drop onto the bottom ring when that happens.
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Repeat the same process for the lower ring. There's enough clearance in the cylinder bolt holes to allow you to wiggle the cylinder a little bit to help work it down over the piston and the rings. Just take it slow and easy and don't force anything if it seems like it doesn't want to go. Be especially careful to ensure that the ring gaps are aligned with the pins and that the piston doesn't get tilted off-vertical as it enters the bore of the cylinder. |
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With both cylinders in place, make sure the head gasket mating surfaces are clean and dry, install new head gaskets, and replace the heads, washers, and head nuts making sure the long nuts are in the same place they were when you took it apart. Tighten the nuts finger tight only and install the cylinder alignment tool across the intake or exhaust ports. I chose to use the exhaust ports since with dual carbs there's no alignment issue on the intake side. No tool? You can use your exhaust manifold if you're sure it's not warped. Check it on a true, flat surface or with a straightedge across the mating surfaces at several angles. Doubly important on a single carb engine. If it's warped, it's very important to true it up. Bob Robertson recommends the use of a large belt sander (big enough for both ports to be in full contact with the flat surface of the belt) with a 150 grit belt or a large bastard file. I decided to buy the tool since, for one thing, I wanted to remove and reinstall the exhaust system as a unit. Although it seems like a lot of money for a machined piece of flat stock with two holes in it that doesn't even say Rotax on it, it's a one-time purchase and doubles as a paperweight. Before starting to tighten things up, rotate the engine carefully to make sure everything moves freely with no binding and no unusual noises.
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Torque it up. Take it up in increments, increasing the torque by about 30 to 40 inch pounds every time around the pattern in the maintenance manual. Make sure you're applying smooth,even pressure on the wrench handle; these engines are sensitive to unequal torque values on these bolts. Also, I've been advised by a mechanic reviewing these pages that one shouldn't use extensions on a torque wrench unless absolutely necessary and even then, use the shortest one possible. And you should only use one hand on the handle of the wrench. He said it wasn't 'super important' but more a matter of 'doing it correctly.' Once you reach the full torque of 195 inch pounds, go around the pattern at least one more time. Recheck the engine for free rotation. After this, you're back in familiar territory. Button everything back up and hang it on the airplane. Be sure to do a careful warm-up and full power static run-up monitoring all the vital signs before you take your handiwork flying. And don't forget that although you don't need to do the full break-in routine after this job, you do need to do all the retorques - cylinder head, intake and exhaust manifolds - after between 1 - 2 hours of operation. |
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