Repairing a stripped/cross threaded sparkplug hole

By Brian Gainey - Jackson, Mississippi - USA

I have been wanting to sit down and actually write an article for Duckworks for some time. Today I was out running my 54' 7.5 evinrude and 58' 18 Johnson seahorse for the first time this year. I guess I got the bug to finally sit down and write about the 18hp spark plug hole repair I did late last year.

Before I get to the details of the repair, I would like to give a brief background on my 58' 18hp Johnson Seahorse. I received this motor for free from my Grandfather. He had gotten it from his cousin 15 years earlier, rescued from under an oak tree, where it had been for an undetermined number of years. Needless to say it hadn't been run in a very long time and was in rough shape.

Rough Shape

When I got it home, I dropped this sad looking little motor into the test barrel and began a couple of basic tests. It had both compression and spark, so I connected my gravity fed fuel source to the carb inlet and began pulling on the blister maker. After 10 or so pulls, nothing, not even a pop. I then carefully drizzled some fuel directly into the intake, and vroooom away she went. I tinkered with the fuel settings for a minute or so and had it idling. I had to shut it down rather quickly due to the water pump, or lack there of, but I at least now knew it was worth spending a dollar or two to see if it could be pushed back into reliable service.

I went to my local NAPA and purchased the water pump/impeller and a couple of spark plugs. I referenced Max's article here on Duckworks to install the impeller, thank you Max and a tip of the hat.

Socket Wrench - Easy Does it.

My trouble started when I went to disassemble everything to install my parts. I found that the motor had been used in South Mississippi in brackish/salt water and put away without any sort of cleaning. I was able to get it apart with several broken bolts and few choice French words along the way. I had to make some "modifications" to get it back together. NO, they aren't pretty and NO, I wouldn't recommend anyone use them.

Now that the impeller housing problem seemed to be resolved, I went to install the new plugs. This is when I discovered that the bottom spark plug hole was stripped out and only had about 1 1/2 threads actually holding the plug in the head. The new plug wouldn't hold in the head, as it is just a little bit shorter than the older plug that came in it. Well I really wanted to see if my "modifications" were going to work and hold up so I put the old plug back in the bottom hole and ever so carefully snugged it up. I completed my test run for the impeller, which worked out just fine.

Later that night I began to do a little research for spark plug hole repair on outboard motors and there isn't much info out there for outboards. Knowing that the plug hole was a standard size, I refined my research to plug size and found several kits. I settled on the Helicoil Save-a-thread. It cost me about $20 plus shipping.

Helicoil Save-a-thread.

This is a very simple and easy tool to use. I was able to make the repair without removing the head. I know that If I ever have to pull the head on this motor it will probably be relegated to the parts motor status due to the salt water exposure I mentioned earlier.

Now on with the repair.

First, lets start with a little safety. Safety glassed are always a must and gloves would be a good idea also. Clamp your motor to a stable work station. A saw horse will do nicely and you could even leave it clamped to the boat, but would be easier on a saw horse. Remove the motor cover. Remove the plug. Carefully turn the flywheel so that the piston for the cylinder you are working on is at the bottom of it's stroke. Now take the tap/threading tool and find your 3/8 ratchet to drive the tool. Now apply a light coat of grease to the recessed grooves, this is to help catch the aluminum shavings you are about to create.

Start the tool carefully and straight in the plug hole by hand, now use the ratchet to slowly begin to cut the threads for the insert. Once the tool has cut the hole, remove the tool and inspect for small shavings that may have fallen into the cylinder or shards at the bottom of the hole that may break off later while running. Shavings can generally be removed with a little compressed air and shards with needle nose pliers.

You are now ready to install the insert. Apply a small amount of RTV silicone of you choice to the threads of the insert. It just needs to be of the high temperature variety. Thread the insert all the way into the newly cut hole. Take the supplied driving tool and stick it into the insert and carefully hit it with a hammer. You are only trying to spread the thin mild steel knurled portion of the insert, so you don't need to get the biggest hammer you own. Cracking the head would surely ruin your day so don't get carried away. Wipe away any silicone that squeezed out. Let the motor sit overnight to allow the silicone to cure.

Now all that is left is to reassemble the motor and give it a test run. I think it took me about 20min to make the repair, and this included actually reading the instructions that came with the tool. I hope this saves someone some aggravation and keeps another one of these great ole' motors humming.



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