Helicoil insert for a stripped spark plug hole

This process was necessitated by a spark plug that popped out of the head and wouldn't stay put when put back in. When a new plug was put in, it would catch but wouldn't stop turning. Stripped threads.

The initial fix was to JB Weld a new plug into the head, in the hope that it would last for the rest of the life of the engine. Didn't work.

Before we go any further: This is not something for the faint of heart. the potential to mess up something really bad is very high. Dailyrider has this advice for someone attempting this: "This is not a novice job. I've been a motorcycle mechanic for over 50 years and it wasn't an easy job for me. Modifying the tool and inserting the helicoils are both very intricate jobs. Take your time and think a lot about what you're doing."

If you don't have a lot of mechanical experience, you may want to consider the alternatives:

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What you need

Perma-Coil 1210-410 kit - This one came from [www.toolskwik.com], but they are available elsewhere. It is the spark plug repair tool for M10 x 1.0 threads.

The actual helicoil inserts don't come with the tool. You can get them from the same place, but you have to order them separately. The spark plug thread is 10 x 1.0 .

Upholstery thread OR a long, thin hemostat clamp.

Grease

Patience – Lots of it.

Modifying the tool

Because our plugs are so small and so far down in the head, you have to modify the insertion tool in two areas. The body has to be cut down somewhat, so it'll even go in the hole leading to the plug. Then the tip has to be cut down, so it will go into the little recess where the body of the plug seats.

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I don't have access to a lathe anymore, so I managed with drills to spin the tool and a grinder or file for cutting.

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Before you even start, prepare the helicoil(s) you're going to use by cementing a 16" piece of upholstery thread to the little insertion tang, as you have to break it off and retrieve it after the coil is inserted. Since it is stainless steel, it isn't magnetic, so there isn't a good way to get it out after you break it off.

(To avoid confusion, the upholstery thread will from here on be called 'string'.)

Tie the string to the tang first, then rotate the knot to the bottom side and glue it there with some silicone seal or something like that; you don't want it to be knocked off when you break the tang off with a screwdriver.

An alternative to this is to get a long, thin hemostat clamp to hold onto the tang while breaking it off.

Bike preparation

Get the bike stripped down basically to where you would be doing a valve check, and then go one step further and remove the gasket and tower on the spark plug hole (two bolts). Be sure to not have both of these off at the same time, otherwise the exhaust cam won't have anything holding it in.

Because the tap has an extension on it that threads into the existing threads, the cylinder needs to be as close to bottom dead center as you can get it. When you're positioning the piston for the cylinder you're working on, turn the engine over until the intake valve rocker arm will just wiggle a little. That way both valves will be closed, but the piston will be well down in the bore, so it won't cause any interference with the Helicoil tap. Since the thread tap is a three-part tool (guide, reamer, and thread tap) there will be a good bit of it sticking down in the bore, and you don't want it running into anything.

Tapping the threads

Next is the actual tapping. Load the tap up with grease to capture the aluminum that is cut. As mentioned, the tap has guide threads, so you will thread those in, and then slowly and carefully tap the threads to the new size. When you are cutting, be sure to keep pressure on the tap and keep it lined up straight so it goes in correctly. Do only one to two full turns at a time, and clean off and re-grease the tap between runs to keep the aluminum from gathering too much.

(This would be a picture of the tap loaded with grease and aluminum if the focus had been correct):

Even though the tap has a built-in guide, always make sure the tap stays centered in the spark plug cavity while you're reaming and cutting the threads. That thread MUST be square with the seat for a good plug seal.

Once you have all the threads cut, put the tap through a couple more times with grease to make sure all the aluminum is removed.

Inserting the inserts

Put the insert on the insertion tool and thread it into the guide so that it is flush with the bottom, as seen in the picture.

Now comes the fun part. Take the insertion tool out and get the thread (string) into the cylinder. One way to do it is to ball up the thread and put a dab of grease on it to keep it bunched up, then put the guide into the spark plug well and use a piece of safety wire modified into a reverse hook to push it down. You don't want any of the thread sticking up, because when you insert the helicoil it will cut the thread, and then all your hard work to get it in is gone.

The next step is to thread the insert into the newly cut threads. Hold the guide with a needlenose or some other similar means while turning the insertion tool. Once the insert is completely out of the guide tool, you will be able to feel the tension release, so you know it is in. Take the guide and insertion tool out and find the top of the string and note the position. It should be right at the top of the threads, and you want it to be a half turn below so it doesn't come unraveled. Once you have the position noted, use the insertion tool to turn it another half turn, and verify that it is now at the other side of the hole.

Next, fish out the thread using a piece of safety wire as a hook, and drape it over the side. Using a long, thin flathead screwdriver, knock the tang off while holding onto the string. If you're using a hemostat clamp instead, use that to hold onto the tang while breaking it off.

Once the tang is out, put the tower back on the spark plug hole, then do the other cylinder, if you need or want to. Once you are finished, reassemble everything (with proper torques) and go for a test ride.

Again, not for the faint of heart, but definitely possible if you take your time and are very, very careful.