helicoil in heads

I hate to tell you, but I don't think it is a good idea to redrill the Helicoil.

If you do, you should use the same diameter drill that you used before you tapped the hole. If you go any bigger you will be taking metal out of the minor diameter of the threads. Any metal taken away from the minor diameter of the threads reduces the load that the thread can support.

Let me give you some definitions so you will understand what I am talking about.


The major diameter is the diameter from the outside thread from one side to the other.

The minor diameter is the diameter from the inside thread of one side to the inside thread of the other side.


The area that supports the load is the difference between the major diameter
and minor diameter. Or here is the math equation:

(major diameter - minor diameter)/2

This is the area that supports the load on a thread. To reduce the stress on a part, you either have to reduce the force acting on it or increase the surface area that supports it. The opposite is true here, increasing the force or reducing the supporting area of a part increases the stress on a part.

Not to mention that it is very hard to redrill a hole that is not crooked and make it straight. Drills like to follow the existing hole and you can make it worse trying to redo it by hand. You will probably make the hole egg shaped or too big trying to fix it by hand. You need a drill press to do it properly.

In your case I think the best bet is to get out the dremmel and grind what you have now to get the bolt to fit. Still a little risky. Then I would recommend tigntening the bolt an extra 3 - 5 ft-lb to make up for the lost clamp load from the cocked bolt if you think that the helicoil is done properly, it should handle it. You don't want to overcompensate and strip out the threads again.

The proper way to correct this is to have the hole drilled oversized and use a "sleeve or plug". Sleeves are typically flat on the outside and press fit into a hole that is slightly smaller than the outside of the sleeve. This definately should be done by a good machinist and not at home. A plug that is threaded on the outside that fits into a hole that is threaded on the inside would be the best way to do this. A sleeve may tend to creep over time with the heating and cooling of the engine and come out. The hole you press it in would have to be cut with a reamer and not a drill to ensure proper fit. Using threads would be best. Basically a Helicoil for a Helicoil, but much fatter. Definately done at a good machine shop on a drill press or mill.

One possibility worth looking into could be to see if you can find a pipe plug with an internal hex drive (hex key). You would want to make sure that you would be able to drill all of the hex drive out and put in a helicoil. If you can't remove all of the hex drive and have gaps to the outside of the helicoil is not recommended.

This again would have to be done in a machine shop. Drill and tap the thread to put the pipe plug in and secure it with a good loctite. Then maching the gasket surface of the head to make sure it is flat and the plug doesn't go above the sealing face of the head and keep the manifold from seating flat to the head. If there was a way to accurately set or countersink the plug .001" to .002" you might not have to mill it flat first, but this would require accurate depth control to insert the plug. Then clamp it in a drill press or milling machine and redrill and tap for the helicoil. This would make the part as good as new but take alot of time and money. Again the diameter of the hex plut would have to be less than the diameter of the helicoil going into it or it is not worth trying.

You probably can get away with the first idea. It is your decision. I'm just giving you some options to think about. I hate to see a T/A head scrapped.