Removing and replacing the head gasket of the Johnson 15-horsepower outboard motor means you have to remove the powerhead of the motor to make the change. The powerhead is light enough to remove by hand. After you have the powerhead off the motor, removing the cylinder head is a straightforward process, as is replacing the gasket. Securing the head bolts is a bit of ballet, dancing from one bolt to another in a swirling pattern several times to tighten them.
Remove the screw connecting the shift lock lever to the side of the powerhead using a slotted screwdriver. Pull the lever from the side of the powerhead. Allow the lever to rest inside the lower engine cover. Use the fuel-line quick-connect to disconnect the fuel inlet hose from the fuel pump.
Remove the mounting screw for the stop switch and ignition module ground leads. Disconnect the stop switch's single-pin Amphenol connector (an electrical connector readily identified by the word "Amphenol" emblazoned on its surface). Take the low-speed adjustment knob, air-silencer cover and finally the air silencer off the powerhead.
Separate the choke knob from the knob detent (the "bump," or mechanical stop, in the choke mechanism that you pull the knob over). Pull the knob away from the lower engine cover. Lift the knob detent away from the carburetor.
Tie a slip knot in the starter rope where it enters the starter's pulley. Untie the knot that holds the starter handle and bumper to the rope. Remove the 3/8-inch bolt at the center of the top of the starter unit and remove the starter from the powerhead. Thread a 3/8-inch, 16-pitch nut on the starter's mounting bolt to hold the assembly together.
Remove the three nuts and six washers -- two washers per stud -- from the lower pan studs with a 5/8-inch open-end wrench. Disconnect the cooling water indicator hose from the powerhead nipple. Remove the three bolts from beneath the powerhead-to-gearcase flange. Lift the powerhead straight up and off the exhaust housing. Remove the exhaust and water-tube assembly from the bottom of the powerhead, then remove and discard the gasket.
Remove the six head bolts that hold the cylinder head to the powerhead. Set the new gasket in place. Set the cylinder head back in place.
Tighten the head bolts, in multiple passes, to between 216 inch-pounds and 240 inch-pounds. Start at the center bolt on the starboard side, then move to the center bolt on the port side, the upper bolt on the port side, and finally the upper bolt on the starboard side.
- Reinstall the powerhead by reversing the sequence of steps for removal.
- "Johnson Repair Manual -- 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007