Repairing a stripped thread can be a costly endeavour, but an insert can be an inexpensive and easy fix.
There are few things that are as frustrating as removing a bolt and finding a little coil of aluminum clinging to its threads. This is an indication that the bolt had been over-tightened at one time, and it pulled the threads out of whatever it was fastening. Until those threads are restored, the clamping power of that bolt is lost. My Honda FT500 recently sprang an oil leak from the camshaft cover, the cause of which was revealed upon disassembly: a coil of aluminum came out with one of the mounting bolts.
The most effective way to restore damaged threads is with a thread insert. A thread insert is a stainless steel coil that replaces the missing threads, while also strengthening them. It is commonly known by the brand name Heli-Coil, much like most everyone calling facial tissue Kleenex, though there are several makers. Thread inserts come in specific sizes, and you need the proper tools to install them. A thread repair kit includes several inserts, an insert driver and a special tap; some kits include a drill bit, while others tell you what drill bit size to use.
The procedure is relatively simple but must be done carefully, as the thread insert must be installed straight. The bolt size on the FT500 measured M6 x 1.0, which identifies it as a 6 mm metric bolt with a 1 mm thread pitch. This is a very common size on Japanese and European bikes, and is usually the size of bolts holding down engine covers. Tightening torque on these bolts is about 90 to 110 inch-pounds, which makes it very easy to over-tighten them without a torque wrench.
With your thread repair kit in hand, the first step is to drill out the damaged hole to the proper size. Although the location of the damaged hole on my bike allowed me to repair it without removing the cylinder head, the top frame tube prevented me from using a drill. To save time, I carefully drilled the hole by hand using locking pliers to grip the drill bit and turn it slowly (yes, this damaged the drill bit, but it saved me a lot of time). Care must be taken not to wobble the drill bit around and make the hole bigger than it need be. Even when using a drill, you should use a low speed and drill carefully and square, and use either cutting fluid or some kind of lube to make a clean cut; I used WD40. One way to make sure you do not drill too deeply is to measure the depth of the hole with a vernier caliper and place a piece of masking tape on the drill bit at the proper distance for reference.
The next step is to tap the new threads. Now, the tap included in a thread repair kit is not a standard size: it’s an oversized tap that has the same thread pitch as the hole being repaired – it is the thread insert that resizes the threads to the correct diameter. When tapping, do not just turn the tap into the hole in one direction the way you would a bolt. You must go in incrementally, about a half turn at a time, then back out the tap about a quarter turn to break the aluminum filings being created during the thread-cutting process. This keeps the cutting edges of the tap clean and prevents aluminum from building up and sticking to the tap, which will produce a sloppy cut. Also use lube when cutting threads for a clean, machine-like finish. Make sure you cut the threads deep enough to accept the insert without it bottoming or tightening on the threads, and don’t forget to blow out the filings.
Once the threads are cut, you can install the thread insert using the driver. The insert has a small tab on one end that hooks onto the driver, allowing you to drive it into the hole. The driver also has a shoulder that bottoms on the surface of the part being repaired so that the insert is installed at the correct height. You’ll feel some tension on the driver as you install the insert because the insert is designed to compress slightly, which locks it in place once it is completely installed.
The final step is to snap off the installation tab at the bottom of the insert by tapping it with a small punch or screwdriver, then retrieving it with a magnet or magnetized screwdriver so it doesn’t interfere with the bolt.
While performing that thread repair I noticed a second hole with damaged threads and repaired that one, too. A subsequent road test revealed that the FT is now oil tight, and we’re both happy.
Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.