This can be a common sight...
This can be a common sight for the Saturday night racer: a bolt or screw containing the first several threads from the part where the bolt was installed. Also, the first five to seven threads in the aluminum are missing. This is not as large a problem as you might think. The repair is fairly straightforward and can usually be accomplished in less than 20 minutes.
Just when you think you have completed all the maintenance and the car is ready for the next race, it happens.
You are tightening the last header bolt or some other bracket or adjustment, and instead of the wrench or socket having that solid feeling you're supposed to get as the bolt tightens, you get the feeling that the bolt is being installed in butter. The threads have pulled, stripped, or have just plain worn out.
Now what?
We all know that problems never seem to occur when it's convenient. Many of the parts used in Saturday night competition were not intended to be assembled and disassembled at the rate demanded by racing. This is especially true with parts that have their roots in passenger cars. This continual assembly and disassembly can take a toll on the threads. Breakage or wear of these components is not as uncommon as we like to think.
You have several options when an internal thread is damaged:
* You can replace the part with a new one, although this is not a viable option for the racer on a budget.
* You can drill out the hole and cut new threads in the part and use a larger bolt. However, this causes problems:
- Now you have a unique bolt installed with the other smaller bolts, and this can cause some confusion during future maintenance. Or it can cause a problem when you get help at the track from other crews not familiar with your car.
- There may not be enough material in the part to support a larger bolt, and the part is weakened to the point of failure.
* You can drill out the part and use a longer bolt with a nut, but this is not always an option, depending on the part and/or the location. You certainly can't use this option with a header bolt.
* You can install a thread insert to repair the thread.
The Tools The tools to fix...
The Tools
The tools to fix the damaged threads are simple, and you probably have the majority in your toolbox now. This is the minimum from a tool perspective. Depending on the location of the part, you may be able to drill the new hole with the part on the car. If that is the case, you will need a hand drill.
What is a thread insert? There are many types of thread inserts, but the one we will be focusing on is the wound-wire type. They are commonly called Helicoils or Recoils; these are brand names and not generic terms that define the product. The simple answer is that a thread insert is a stainless steel wire with a diamond-shaped cross section that has been coiled into spring. It is this spring that is wound into a threaded hole, and it is this insert that the bolt is screwed into once the insert has been installed into the damaged hole.
The process of installing thread inserts is not as difficult as it may seem. The manufacturers of the inserts have made the process very easy. The tooling required is minimal. In fact, I would be willing to bet that most racers have the majority of the tools already on hand, with the exception of the installation tool, which comes with most installation kits.
The process requires some care and patience. The result is usually a thread that is stronger than the original, and it is always a good thing if the repair is better than the original. It has been said that in order to finish first, you must first finish. Well, we might also say that in order to finish first, you have to first get the car ready to finish.
What is shown here and on the following page is a step-by-step pictorial demonstrating the process for installing a thread insert.
The insert is a very simple...
The insert is a very simple part. This view shows the drive tang. The drive tang will engage a slot in the installation tool, and then it's merely a matter of screwing the insert into place. Once this insert is installed, the new thread will be stronger and more durable than the original thread.
The damaged hole and the thread...
The damaged hole and the thread insert. The hole will be drilled out just a bit larger than the original hole. This will be followed by tapping new threads into the hole. All chips will be cleaned out, and then the new insert will be installed and the drive tang removed.
Double-check the drill bit...
Double-check the drill bit for the correct size. The kit called for a 17/64 bit; the decimal equivalent is 0.265. Make a quick double-check to make sure you have the correct drill bit prior to drilling the hole, as a hole that is too big creates an additional problem.
There will be times when you...
There will be times when you will not have the luxury of using a drill press or a mill to perform the drilling operation. You may have to accomplish the drilling operation on the car. If that is the case, you need to make sure you drill the hole straight and make every effort to prevent the resulting chips from contaminating any of the systems on the car. In order to drill the hole straight, you may have to fabricate a drill guide to assist you in drilling. Remember to use a little cutting fluid or some light oil, such as WD-40, to help lubricate the drill.
Once the hole is drilled,...
Once the hole is drilled, you should use the appropriate-sized countersink to deburr the hole and help the tap start straight. This will only require a slight kiss of the countersink to the hole. Now is a very good time to make sure the hole is as clean as possible and free from chips.
Exercise a great deal of caution...
Exercise a great deal of caution to ensure that the tap is going into the hole straight and square to the surface of the part. Failure to make the threads straight will result in the insert being installed crooked, and this will allow the bolt to be installed at an angle. The resulting binding will weaken the final assembly due to the uneven clamping forces of the bolt. This is the point in the process where you need to make sure you take your time. Remember to use an adequate amount of cutting fluid or light oil to keep the tap lubricated and help any chips flow out of the work.