Preface

While this article may be redundant to some readers, I have tried to gear the context to those who have never had the need or had anyone show them how to properly install a heli-coil. Heli-coils are very useful when threads have been damaged and replacement of the assembly would be too costly That being said, installing heli-coils becomes a double-edged sword. When done properly, heli-coils can save you time and money in the repair of damaged threads. When improperly done, the damage to a component may result in the part needing further repair or at worst, replacement. While this text was written as a guide for the repair of carb bowl screws, the reader may find the general information helpful in thread repair on future projects.

Heli-Coils

A heli-coil is a replacement for threads that have been damaged through improper threading of hardware or over-tightening of hardware resulting in the stripping of the threads within the component. They can be used in almost all types of metal that allow for re-drilling and re-tapping of the hole. While not requiring any real skill in installation, several steps should always be considered and followed while installing one. In some cases, re-tapping the hole might be an option, however, almost always the required material needed to be removed to properly re-tap a hole results in an over-sizing of the hole, thus requiring a larger hardware configuration. This option is often used in applications where size isn’t a factor. In engine repair however, often as not there is little or no tolerance for an increase in hardware size.

The type of metal to be repaired on most motorcycle engines is aluminum. Because of basic engine designs, steel or hardened hardware is used to assemble components. This difference in materials doesn’t allow for too many removals or installations before wear occurs within the thread hole. Even with the best of conditions and the best of care, wear occurs. Once started, the wear turns into a bind. Depending on how dense the aluminum has been cast, some components may allow for several uses while others may only allow for one or two uses and the threads will become almost un-usable. In the case of carb parts, the metal is very soft. Maybe because of overall cost of manufacturing, carb bodies usually don’t allow for too many disassemblies before problems with threading occurs.

Rule One: Is A Heli-Coil What I Need?

If part replacement is too costly or unavailable, consider heli-coiling. Often as not, the requirement to heli-coil something doesn’t show up until too late. When removing hardware, if the bolt is tight throughout it’s travel coming out, you can almost bet the threads paid the price. 

Rule Two: Is This Something I Can Do?

Installing heli-coils require no special tooling. There are several companies that make heli-coil kits. Included in the kit are the coils, an inserting tool, the proper tap required to re-thread the hole, and a tool for knocking off the hook on the coil used for inserting it in the hole. It’s best to have the hardware you need the coil for when you go to the store. On the kit box there is printed the proper drill size needed to drill out the damaged threads. Other than the kit, the only tool you need is a hand drill or the use of a drill press.

Rule Three: Don’t Be In A Hurry

Patience is a virtue when installing heli-coils. Not everyone is a machinist. Making sure you drill the hole straight and clean will pay off in an easy and tight installation of the coil. When drilling aluminum, it takes almost no pressure on the drill to get results. If you have access to a drill press it’s best to use one. Hand electric drills work well if you can drill straight. Always use clean, sharp drills. A dull drill will almost always over-size the hole, making for a sloppy coil thread. Have a firm hold on a hand-held drill motor. Drilling soft metals often causes the drill to “chew” its way down or bite to one side causing an oblong hole, so for using a drill press if you can.

Rule Four: Work On Junk Before You Work On The Part.

It’s a good idea to practice on a junk piece before you try your hand on the part your trying to repair. For the cost of a coil or two, it’s money well spent to try and install a coil on a work piece before you do the actual installation of a coil for real. Once installed, coils are hard to remove without damage to the part you are trying to fix.

Working On The Carbs

I’m forgoing the wording on how to remove the carb bank. Once out, invert the bank and remove the bowls. It isn’t necessary to remove all of the inner bowl components unless you feel you need to. Personally, I left them intact. With being aware of the drill-out chips I used compressed air to blow things out after each hole. In my case, the holes had already been re-threaded once before and the bowl screws had been replaced with 10/32NF screws. This was the size I had to buy coils for. I’m sure the stock screws were smaller, but again, if you plan on keeping with the stock size, take one of the screws with you to the store so you and the salesman will be talking the same size.

I used a drill press to ensure the holes were drilled straight. I placed a small sprit level across one of the bowl edges to ensure the hole was straight down.

After all 16 holes were drilled, I proceeded to tackle the job of tapping out the holes.

In drilling out the holes, two of the four holes in each bowl are just above protrusions cast into the body. Don’t allow the drill to hit them. The caution statement has one other facet. The kits only come with one style of tap. In order to properly finish out the close-drilled holes you will need a “bottoming tap”.

In tapping aluminum, TAKE YOUR TIME AND WORK SLOWLY. Because it is so soft, it has a tendency to “pack” in the tap. Don’t simply tap straight through. 

Work the tap back and forth with each ¼ turn. If allowed to, a tap will not only make a good thread going down, but wipe out the thread on the way back out. By “rocking” the tap you will help keep the cutting threads clean. If you are threading a blind hole, remove the tap after every two or three turns to “blow out” the chips from the cut.

Once you have tapped all of the holes out, take your time to insert the coils. Shown above is the insertion tool supplied with the kit. The depth set is adjustable for the depth required to flush-set the coil. Because of the ridge on the carb body, this gauge will have to be set somewhat higher than needed. If the holes are properly tapped, the coil will thread down smoothly with a small amount of pressure. In the case of the bowl screws, the coils came the right length for the hole. You should not have to cut any of the coils to get a flush fit.

Shown above is the tool supplied with the kit that is used to remove the cross-bend in the end of the coil used to set it. Once the coil is in place, insert the tool and lightly tap it with a small hammer to break the tab off.

NOTE: If installing a heli-coil in a blind hole, extreme care should be taken to ensure you have enough depth in the hole to properly remove the tab. If not, run the heli-coil to the bottom of the hole and insert a second coil. This practice isn’t recommended, as it’s not easy to remove a second tab without damage to the lower insert. The purchase of a longer coil is the right answer.

Now that all of the coils are in place, take one of the screws and go over each hole to ensure there are no issues with the new threads.

Hopefully, this text will help you with the repair of the stripped holes on your carb bodies. The technique of heli-coiling is one that will be of service to you in the future. I’m certainly no writer and I have much to learn as a motorcycle repairman/rider. While there are many machinists who may know a better way to do this task, these are the ways I have used in the past with no problems.

If anyone wishes to add or modify this text to better it’s content, by all means, do so. There’s always more than one way to skin a rabbit.  Good Luck.  Larry Smelley. CWC971