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How to Install Outboard Motor Head Gaskets

by Will Charpentier
A new head gasket can prevent a smoking outboard.

A new head gasket can prevent a smoking outboard.

Exhaust smoke leaking from your outboard's top cowling is never a good sign. It usually means that you need to install a new head gasket on your outboard motor to prevent exhaust gases from escaping from between the cylinder block and the powerhead block. The procedure is straightforward and, as long as the gasket is sealed and the cylinder block is properly tightened to the correct torque, it should alleviate the problem.

1.

Remove the cylinder block bolts with an adjustable wrench. Lift the cylinder block from the powerhead. Remove any gasket sealing compound residue from the surface of the mating surfaces of the cylinder block and powerhead block, using a plastic scraper.

2.

Spread a thin layer of gasket sealing compound on the top and bottom sides of the new cylinder block gasket and position the gasket on the cylinder block, using the alignment pins on the cylinder block to position the gasket.

3.

Apply a very light coat of clean engine oil to the threads of the cylinder block bolts. Use the alignment pins on the cylinder block to position the cylinder block on the powerhead block.

4.

Thread the cylinder block bolts into their bores and tighten them with your fingers. Tighten the upper bolt on the port side of the block to 9 foot-pounds. Use a crossing pattern to tighten all of the cylinder block bolts to 9-foot pounds, ending at the upper, starboard bolt. Continue to tighten the cylinder block bolts until the torque for all bolts is 19 foot-pounds.

Items you will need

  • Adjustable wrench
  • Plastic scraper
  • Gasket sealing compound
  • Clean engine oil
  • Torque wrench

Tip

  • You should conduct a compression test after you install the new gasket. Remove the No. 1 spark plug and install a compression gauge in its place. Crank the motor over three times, without starting it, and record the highest compression reading for the cylinder. Repeat for all cylinders. The recorded readings for all cylinders should be within 10 percent of each other. If the readings are not within 10 percent of each other, the powerhead needs a complete overhaul.

Warnings

  • Disconnect the negative cable of your battery before performing any maintenance work on your outboard motor to prevent electrical shock or accidental starting. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from your battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.
  • Whether you work on your outboard motor on the boat or on a storage stand, remove the propeller nut with a wrench. Slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before operating an outboard out of the water during maintenance or long-term storage is an invitation to a propeller-strike injury, which can maim or kill.

References

  • "Evinrude Repair Manual -- 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007
  • "Mercury Outboards Repair Manual -- 40 to 115 HP, 3- and 4-Cylinder 1965-1989"; Seloc Marine; 2007

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits

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