Helicoil insert for a stripped spark plug hole
This process was necessitated by a spark plug that popped out of the head while going down the road. 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:
1. Replace the head. If you can get a completely assembled used head in good condition, you should be able to replace the head in about the time it would take to helicoil both plug holes, and with a whole lot less aggravation. A new head gasket, an o-ring for the coolant stub to the thermostat, and some coolant should be all you'd need.
2. Find someone else to do it. It might be difficult, though. You'd have to find a good mechanic who is also a machinist, or a machinist who is also a good mechanic, and there are fewer people like that all the time. The cost of labor has gotten so high that it's no longer practical to make parts fit, as in the days before the Japanese started making bikes where every part made for a certain bike would fit every bike that the part was made for. People used to have to hand fit every part, but that generation is dying off now and it's harder to find someone like that.
Probably the best place to find someone to do the job would be a shop that specializes in old European bike repairs. Unless you can't find a good used head, it probably wouldn't be cost effective. At today's shop rates, both helicoils would likely run you over $500. The helicoil does make a stronger than stock hole, though; the BMW factory used to helicoil all their plug threads.
What you need
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. This one was done by putting the tool in a drill, spinning it around, and using a grinder to remove the material. A lathe would be much better, if you have access to one.
Preparing the helicoils
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.
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. Only do one cylinder at a time.
The tower completes the plug cavity from the head to the valve cover. It's a part of the exhaust cam bearing. The two cam bearing bolts hold the bearing with the tower to the head. There is an o-ring between the tower and the head, but it should be ok to reuse.
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.
If you're doing both cylinders, don't forget to position the crank for that side in the same way.
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.
The helicoil insert is screwed into the outer part of the insertion tool by the inner part of the insertion tool, which is like a very long spark plug thread with a very shallow hook on the bottom. This grabs the insertion tang on the bottom of the helicoil.
The helicoil is screwed into the outer part of the insertion tool until it's at the very bottom thread. Next, you back out the center part of the insertion tool and use the reverse hook to poke the upholstery thread well down into the cylinder (while keeping the outer part of the insertion tool tight against the plug hole). When you're sure you have it well clear of getting trapped by the helicoil, you're ready to continue.
One way to do this 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. (To make a reverse hook, make a hook to pull with, then take 50% of it and bend it back the other way. This makes a hook to push with.) 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.
This next part is very, very critical: Screw the center part of the insertion tool in until it touches the insertion tang again, then you MUST hold the outer part of the insertion tool tight against the threads you cut in the head (in other words, make sure it isn't up on the shoulder of the plug recess). This is to keep it from skipping a thread. Use a needle nosed pliers to hold it down tight. Screw the helicoil in until tension is released on the outer part of the insertion tool; this indicates the coil is completely out of the outer part of the tool. The last coil of the helicoil should be level with the plug seat.
Once the helicoil is completely out of the outer part of the insertion tool and in the head, unscrew the center part of the insertion tool and remove the outer part of the insertion tool. Now look to see where the last thread of the helicoil ends. This is not easy to determine, as it's very hard to see down in that dark hole, even with a good flashlight. You want that end of the helicoil to be approximately 1/2 thread below the surface of the plug seat. This is very important. This will keep it from unscrewing with the plug. You also don't want to run it in too far, because you don't want the bottom thread of the helicoil to be clear of the threads in the head, which would tend to cause it to glow from combustion temperatures and cause pre-ignition.
Once you're sure you have the helicoil positioned correctly, fish out your upholstery thread with the hook (leave plenty of slack in it) then break off the insertion tang and retrieve it with your upholstery thread. To do this, use a long, thin, flathead screwdriver and 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 the proper torque. The torque on the cam bearing (tower) bolts is 104 inch-pounds (8.7 foot-pounds). 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.
And a final word from Dailyrider: "Working in that deep dark hole the plugs are in is no fun. You can't see anything, and it's very hard to feel anything; there's just no room at all. I would rather never do another one myself. It is a very tough job."