Engine Decarboning: Reassembling the Pistons
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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Reassembling the pistons begins with buying or making a special tool to hold the needle bearing assembly together while reinserting the wrist pin into the piston. You can spend big bucks for the special tool (which will require you to remove all four circlips which will, in turn, require you to remove the fan tower to get at the back of the mag end piston to reassemble that circlip. Or, you can spend pennies for a short length of 3/4" foam backer rod which does the job great and allows you to leave the mag end circlips in place. Backer rod is sold in construction supply and hardware stores usually in the same area as weatherstripping and caulk. It's used to fill large voids in structures before the application of seam fillers or caulk. Made your choice? OK. Photo shows cutting the backer rod to fit between the insides of the wrist pin bores on the underside of the piston. After cutting to length, roll the slug of rod between your hands to break down the foam cells a little bit. This will make it easier to remove from the piston later. BIG thanks to Tom Olenik of Olenik Aviation for this idea.
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Make sure you've got the parts for the correct piston. Begin the reassembly of the bearings by placing as many of the 31 needles (count them again before you start to make sure all are present and accounted for) into the bore of the connecting rod end as will stay put of their own accord. A light coating of two-cycle oil will help them stick to the rod and to each other. Note that in these photos the mag end piston is already assembled in the background. You do want to do them in this order so that you have the maximum room to work as you insert the wrist pin from the PTO side of each piston. Also, by turning the crankshaft you can move the rod you're working on to the top of it's stroke to give you even more room. |
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Next, insert your foam slug into the bore being careful not to disturb the bearing needles already in place. Then insert the remaining needles around the outside of the foam slug. They should all fit with a slight space less than the thickness of one needle after the last one is in place. Check carefully to see that all the needles are side by side and in contact with the bearing surface of the rod. The ends of the foam should be projecting beyond the end of the needles just enough to hold the needle thrust washers in place. These washers are symmetrical and can be reinstalled without regard to previous position. But if you care, the side of these rings that is slightly shinier than the other is the side that faced the ends of the needles before disassembly, that is, it faced toward the connecting rod.
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Now for the slick part. Make sure you've got the right piston and that it's oriented the right way: there's an arrow and the word "aus" embossed in the piston crown. The arrow points to the exhaust side of the engine. The circlip you didn't remove is on the Mag end side of the piston. OK? Carefully bring the piston into alignment with the connecting rod. As you work the piston into position, with the connecting rod end and bearing assembly in alignment with the wrist pin bore,be careful that the needles and spacer rings stay in place and all in alignnment. Check by looking into the piston pin bore from the PTO end; you should see the end of the foam slug centered in the hole and you should not see any needles. If you do, something has shifted out of position. When it looks good, hold everything in position and start the wrist pin into the bore and watch from the underside of the piston to make sure that the pin slips through the first ring, then into the needles without displacing any needles or pushing them out the other side of the connecting rod end. The pin will be pushing the foam slug out the other side of the piston as you go. It may help to rotate the pin as you push it through. Once the pin is into the needles, if the far ring shifts position or if any needles move, you can probably coax them back with a small screwdriver, wooden skewer, or other similar tool. When the pin has passed through the needles,the other ring, and has started into the mag end side of the piston, you're home free. You can then reach around back and pick the foam slug out of the bore (getting it through the circlip is why you crushed it before) and finish pushing the wrist pin home. The photo shows the magic moment when the wrist pin is almost in and is pushing the foam plug out the mag end. Piece o' cake right? |
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There's one more tricky part to this - putting in the new circlip. Wear safety glasses. Position a spotter in the room (also wearing safety glasses) to watch for where the damn thing lands when it whings out of your grip. Place one end of the wire clip into the groove closest to the bottom of the piston - when installed, the gap in the circlip must be pointed toward the bottom of the piston. With the thumb of one hand press down and toward the center of the wrist pin bore. With the other hand push on the ring with a large screwdriver just where it crosses the edge of the bore as in the photo. As with disassembly, be careful not to gouge or scratch the surface of the piston. With practice, and luck, a segment of the ring will slip into the groove. Then move the screwdriver around to the new place where the ring crosses the edge of the groove and push there. Another section will slip into place. Repeat till it has all snapped into place. Now do it all again on the other piston then turn the engine upright and remove all rags and padding in preparation for cylinder reassembly. |