Due to the fact the outboard engines are typically small, compact systems, they rely heavily on one or two pistons to produce their power. When a piston goes bad or loses compression, it needs to be replaced. While the process does not involve extremely technical steps, piston replacement does require following disassembly and assembly steps correctly. Otherwise, the piston won't work properly when the owner or mechanic tries to run the engine.
Disconnect the spark plug wire and electrical connections to the outboard engine. Use a socket wrench to loosen and remove the nuts that hold the engine cylinder onto the assembly. Disconnect the exhaust pipe as well using the same tool or a crescent wrench.
Carefully pull the cylinder off the piston and crankshaft inside to expose the piston. Place a shop rag around it so that the piston and crankshaft arm won't flop against anything.
Use a circlip plier to remove the piston gudgeon pin circlips on the side of the piston being replaced. Put them in a safe place. Insert a piston gudgeon tool into the center of the piston and screw on the stop at the other end as it exits the piston. Twist the handle to pull the gudgeon pin out from the middle of the piston
Remove the old piston from the crankshaft. Pull the gudgeon pin free from the old piston. Set it aside. Look at the top of the new piston for the exhaust marking -- it looks like a little arrow. Position the piston so that the arrow points to the exhaust side of the cylinder when it is installed. Insert the gudgeon pin into the side of the new piston.
Place the new piston in position on the crankshaft arm after inserting a new piston bearing inside the crankshaft arm which the pin will penetrate. Use the gudgeon pin tool through the piston to pull the pin through the piston, bearing center and crankshaft arm, securing all three together.
Remove the gudgeon pin tool and insert new circlips into the side of the new piston to keep the pin in place. Carefully install the new piston rings onto the new piston and position them to align with the piston ring slots on the piston.
Grease the engine cylinder with engine oil. Carefully position the cylinder back onto the engine and slowly insert the attached new piston into the cylinder. Squeeze the piston rings into place in the piston slots as the piston goes into the cylinder. Push the cylinder home into the engine block.
Tighten the cylinder with new nuts and washer using the socket wrench and sockets. Check the tightness of the cylinder nuts with a torque wrench. Tighten the nuts to factory setting torque per the outboard engine repair manual. Reinstall the spark plug wire to the spark plug in the cylinder head.
- Make sure the piston is positioned with the arrow on the piston head pointing to the exhaust. Installing a piston upside-down will cause the engine to run poorly as the piston vents won't be aligned with the cylinder ports inside the engine.
- If you can't find the appropriate engine tools at your hardware store, motorcycle shops have a variety of basic engine repair tools, such as circlip pliers and gudgeon pin tools, that work well for small engine repair.
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