Thread Repair 101

In June of 2004, Dwayne Verhey (aka Torch) from the FJROwners online forum wrote this Web page describing how to repair damaged threads.

Forgot to reset the torque wrench after tightening the head bolts, eh? Now you've gone and stripped out the threads from that nice expensive aluminium casting...

Not to fear. There are ways to repair that head. The cheapest and easiest is to oversize the bolt hole and re-tap. But sometimes that isn't an option. Such as when the bolt is a special design...

Now you need a thread replacement kit. The most famous brand is Heli-coil. The old hole is drilled slightly oversize and a spring-steel thread insert is installed.

Here a 6mm thread is being replaced. First, the hole is drilled out to the specified diameter (1/4")

Next, the enlarged hole is tapped with the special Heli-coil tap. This tap comes in the kit and is the same thread pitch as the original 6mm, but the diameter is slightly increased to accomodate the enlarged hole.

The spring steel insert is installed into the hole using a special tool, also supplied with the kit. The design of the tool has changed over the years. Two different designs are pictured here.

Longer threads can be made by stacking inserts, installing the first one deep into the hole and then adding a second above it, but the standard length is usually sufficient in aluminium.

Both designs have one thing in common: they wind in the insert by driving the bent tang on the bottom. This tang is scored (visible at the top in this picture) so it can be broken off later if need be, but leave it in place until the insert is installed.

Since the insert is a spiral (kind of like a slinky) twisting it in contracts the diameter slightly. When the tool is removed, the spiral expands to grip the hole tightly.

Here the insert is installed with the tool into the prepared hole. This particular tool is designed to ensure the insert is inserted at least flush before the tool can be removed from the work.

Some mechanics recommend a little red (permanent) Loctite to prevent the insert from ever coming loose. If you go that route, use it sparingly and wait for it to set before installing your new bolt. Just in case a little leaks through the spiral...

Here's the insert installed into the hole. If you look closely you can just see the drive tang still in place at the bottom of the insert. Make absolutely sure the insert is deep enough (at least flush with the top of the casting) before proceeding to the next step.

Finally, a square ended drift is used to snap the drive tang off inside the hole. One quick rap with a hammer is all it takes.

Getting the tang out can be a little trickier in a blind hole. Since the engine was still in the bike, turning the part upside down and shaking really wasn't an option. A tiny magnet might work, but often a greasy Q-tip is the simplest solution.

The special design of this bolts underscores the need to repair the existing hole in this case, rather than oversizing.

The kits come in a wide variety of sizes. While big expensive professional kits are available that cover a range of sizes, the backyard mechanic is probably better off buying small individually sized kits as needed.

They are still pricey, but at least you don't have to mortgage the house to afford one! Each kit comes with the tap and insertion tool appropriate for the size, as well as a few inserts to start off with. Extra inserts can be purchased separately as needed. Tap handles are not included. If you do not already have a tap and die set, you will need to buy a handle suitable for the size tap that came in your Heli-coil package. Oddly enough, sometimes the tap and the insertion tool need different size tap handles!

A word about sparkplug hole repairs: The kits for sparkplug holes are slightly different. Instead of a spiral, the insert is solid. They cannot be stacked: the correct length that corresponds to the sparkplug reach must be selected. They are available in short, normal, and extended reaches.

A special tap is used to simultaneously ream out the hole to the proper size and cut the new threads. The same tap is used to thread the insert into the hole until flush. The inserts come pre-coated with a thread sealant compound to make them gas-tight under compression.

The outer end of the insert is knurled. The insert is expanded with a hammer and the special drift, driving the knurls into the aluminium and locking the insert into place permanently.