Helicoil Installation, December '04

The Problem

My car was leaking oil out the valve cover gasket and I first assumed the leak came from where the head and the upper timing case cover meet. (The valve cover extends over both the head and the timing case cover that surrounds the timing chain). But when I took a closer look, I found that almost all of the valve cover bolts were loose and spun in place. Over 13+ years I had managed to strip them one by one and had reached the end of the line. Unlike my first car ('76 Rabbit), these valve cover bolts have a shoulder that stops compressing the valve cover gasket once it makes contact with the head. In simpler terms, once the bolts are finger-tight, the gasket is fully compressed. Tightening beyond that point only prevents the bolt from coming loose. And tightening too far only strips the threads inside the aluminum head.
What do you do when you strip out threads in aluminum? You either drill and tap a larger bolt side (and use a larger bolt), or install an insert. One of the most popular varieties is called a Helicoil™, which looks a lot like a spring made of square wire.
Helicoil Insert
Basically you drill a larger hole, tap new threads, screw a Helicoil into the new hole, then screw the bolt into the Helicoil. The insert is sized and shaped such that it digs into the aluminum and will stay in place when screwing in a bolt (or latter removing it).

Helicoil Installation

The M42 valve cover uses M6 bolts so you need to pick up an insert kit for that size like the one shown below.
Helicoil Kit Outside Helicoil Kit Inside
My kit included ten inserts, a tap, and a special tool that allows you to drive in the insert as well as compress it for easier installation. However, you need several more tools to complete the job. The tools I used are shown below.
Tools Used
Not included in the kit are a 1/4" drill bit, a sturdy low-speed drill, tap wrench for the included tap as well as a smaller one for the installation tool, and finally a pin or solid punch and small hammer. You also need a shop-vac to pick up metal shavings after each step.

  1. The first step is to drill out the existing hole. The hole needs to be both deeper than the bolt as well as long enough for the tap to cut full threads to the depth of the insert. (The first 3/16" or so of a tap doesn't cut a full thread). I marked my drill bit with tape to indicate the proper depth. In practice I found I could feel when the bit transitioned from making the hole wider to making it deeper. That turned out to be about the same depth as I had marked. Carefully cover the valve train with a T-shirt or rag. Then drill out the hole being very careful to position the bit perpendicular to the head.
    Drill Depth Stop Drill Out Hole
    Carefully vacuum up the metal shavings while remembering that the shop-vac would rather suck up your T-shirt or rags.

  2. Next you need to tap the hole. Liberally apply thread cutting oil to the tap each time you start a new hole, or after cleaning off metal filings.
    Lubricate Tap
    I found grabbing the T-handle of the tap between my index and middle fingers then using only my wrist to rotate the tool worked quite well.
    Tapping Threads 1 Tapping Threads 2
    Keeping the tap perpendicular is critical during the first few turns. Later on the tool will align itself. After three or four turns, back the tap out, clean off the filings, and re-apply oil.
    Tap Filings Clogged Tap
    If you keep driving in the tap, it will eventually start giving you resistance as the filings clog up the flutes. If you go that far, those same filings will mangle your new threads as you back out the tap. The basic rule is that it is never too early to back out the tap, clean it, and go again. Continue to cut threads until the tap bottoms out in the hole.

  3. Now you are already to install the insert. Thread the insert onto the end of the installation tool. There is a little tang that mates with a ridge on the end of the tool so you can screw in the insert. Gently screw the insert into the compression tool until it is almost ready to come out the end of the tool.
    Insert on insertion tool Insert in compression tool
    Now place the whole thing over the hole and hold the compression tool while gently screwing the insert into the hole.
    Tool Assembly in place
    While the insert is still inside the compression tool you can feel it trying to twist. When that force goes away, the insert is installed. [You don't technically need the compression tool, and your kit may not include one, but I found it quite nice to use and much less likely to mangle the insert]. When done, the insert should be entirely in the hole (but just barely). Note that you cannot back out the insert so be sure to stop in time.
    Insert in Place
    The above picture shows an installed insert with the tang still in place. (The tang is on the bottom end of the insert, despite how the picture may appear).

  4. The last step is removal of the tang. The tang comes scored (like a piece of glass). Insert a pin punch into the hole and give it a quick tap with a (small) hammer.
    Punch Out Tang
    A staccato strike will take the tang right off, a mushier blow will just bend it. Vacuum up the tang and you are ready to screw in your bolt. If you don't remove the tang then the bottom end of the insert will be mangled when you insert the bolt.

Afterword

In the case of my valve cover, I screwed in each bolt (with the cover removed) and tried to torque them to spec. This identified the stripped bolts, which totaled eight, and I installed inserts for each. After dabbing some RTV on the gasket (where the timing cover meets the head), and replacing the valve cover, two more bolts spun. One of those was along the lower edge so I installed an insert with the cover on (and left the other loose, for now). In order to make the drill long enough, I could only chuck the last 3/16" of the bit, hence my advice to perform installations with the cover off. My car made the 800 mile trip home for Christmas without leaking oil.
If you have never drilled and tapped threads before, I recommend practicing on a piece of scrap before attacking your engine, but the process is pretty straightforward. Perhaps the most difficult part is overcoming the fear of drilling your head.