Strippers can be expensive. Stop thinking rap stars, lap dances and sports celebrities. We are talking about a “stripper” in terms of a hole that has had its threads stripped out or damaged. In addition to being expensive to repair, strippers also require a great deal of time and usually evoke a great deal of frustration. Fortunately, understanding the history of the materials being threaded along with the science of inserts can help you avoid encountering a stripper or, at least, be prepared to select the correct parts to repair a stripper. Done right, you’ll never have a thread failure in the same hole again.

Text and Photos by Michael Ferrara // Illustrations by Paul Laguette

Hard Times

Forty years ago, the world was a harder place. Your grandparents weren’t lying to you. Cast-iron alloys and steels were the basic building blocks for just about anything. On the automotive side, grey cast iron was the “go-to” material for engine blocks, cylinder heads, manifolds, housings and heavy brackets. Steel was the material of choice for smaller brackets, fasteners and the frame and body of the vehicle. While the mechanical properties (strength, stiffness, impact and corrosion resistance) varies immensely across different grey cast iron and steel alloys, they all have one thing in common. For the most part, these materials, when used on components that have threaded holes, are all on the “hard” side with Brinell Hardness ratings (HB) in the 150 to 300 range (tool steels can get up to 900 on this scale). Being hard in nature, grey cast iron and steel will tend to have exceptional thread strength and thread retention capabilities as long as these materials are protected from corrosion. When thread damage does occur in cast iron and steel, the most common practice is to drill out the existing threads and tap the hole to accept a larger fastener. In many cases, this solves the problem. When the same size fastener must be used, an insert can be installed to save the day.

Going Soft

Today, it’s not just “emo” millennials that are softer. We live in a softer world. The materials that have replaced grey cast iron and steel are relatively so . The world has moved from the Steel Age to the Aluminum Age. Pure aluminum will only hit 15 on the HB scale, whereas aluminum alloys can range in hardness from 20-to-150. The aluminum alloys used on most of the sand cast components in a vehicle are typically in the 45-to-65 range in terms of hardness. The highest hardness cast-aluminum alloys are used in block and cylinder heads. These will typically have a range of hardness from 75-to-85, about the same as 6061-T6. Only 7075-T6 aluminum alloy has a hardness of 150 which is on par with grey cast iron.

Hard Bolt Meets Soft Threads

While the materials where threads live have become softer, the materials used for fasteners have in many cases become harder. In fact, some of the super-alloys used in high-performance head studs and rod bolts have hardness levels approaching that of tap (every try to drill out a broken tap?). As a result, it’s no surprise that strippers are more common than ever before. Since most of the threads on today’s vehicles are made of materials with just one-third to one-half the hardness of grey cast iron, it’s no wonder that more threaded holes experience thread damage than ever before.

It’s been said, “If a threaded hole in a softer metal uses a fastener that is installed and removed on a regular basis, a harder-material insert is a must.” Unfortunately, the cost and time involved with installing inserts at an OEM level means that it will never happen on a large scale. There may be a few key, high-stress locations in an engine block or suspension component, but 99 percent of all the threaded holes will not come from the factory reinforced with an insert.

The Solution

If every single aluminum component in the world could be manufactured from 7075-T6 aluminum alloy (which has equal hardness to typical grey cast iron alloys), the number of strippers in the world could be brought back down to the Steel Age numbers. Due to the high-cost of this material and the need to cast certain parts, this will never happen. Instead, the solution for stripped threads are inserts.

Insert Tech 101

Thread-repair inserts come in a variety of materials and in a myriad of designs from domestic and overseas manufacturers. Timed thread body inserts, wire coil inserts and keyed inserted make up the three major families of thread inserts you’ll likely encounter. Each design has its merits and may be superior to another based on budget, installation time, maximum O.D. permitted and the necessary strength of the repair or upgrade. So which type of insert is best? It all depends on the grading criteria and the application. Here’s an overview of the three main types in order of popularity.

HeliCoil Helical Coiled Wire Inserts

Band-Aid is a brand synonymous with bandages, while Heli-Coil is a brand synonymous with thread repair inserts. Encounter a stripper and the solution is often touted as ,”just HeliCoil it.” HeliCoil and the similar designs inspired by this design are available from a number of other companies. It’s the thread-repair solution most likely to be found at local auto parts stores.

HeliCoils were originally made from square, stainless-steel wire wound so that the wire had a diamond profile that would fit into the repair threads. When installed into the larger repair threads, the wire forms a set of threads engineered to meet the original dimension of the stripped-out hole. The repair threads for the HeliCoil insert are made by using a Screw Thread Insert or S.T.I. tap. This is the same type of tap used for some other inserts. An S.T.I. tap uses a simple but somewhat misleading naming convention. An S.T.I. tap is actually about 14 percent larger in diameter than a standard tap of the same designation (M10x1.25 versus M10x1.25-S.T.I.).

Here’s how it works for an M10x1.25 thread repair. First, you would drill the hole with a 10.25mm drill (a standard M10x1.25 thread would use a much smaller 8.75mm drill). Then you would use an M10x1.25-S.T.I. tap. This S.T.I. version of the tap will have a major diameter of up to 11.787mm, whereas a standard M10x1.25 tap is about 1.5mm less in diameter. Once the hole has been drilled, tapped and cleaned for the repair insert, the HeliCoil insert is screwed into place with the installation tool. Once the insert is located in the general vicinity of the desired location, the tab is broken off to secure the insert in place.

In addition to being the most popular and readily available thread insert, HeliCoils also require the minimum amount of oversizing to the hole (same as TIME-SERT), provide a corrosion resistant thread material, and deliver the least expensive solution (about the half the cost per insert compared to TIME-SERT and just one-fifth to one-twenty-fifth the cost of Keenserts). On non-blind, through-holes, HeliCoils can also be installed from the backside of the hole making it the only solution in some situations.

While HeliCoils may be one of the better choices for repairs that are not under extreme stress, strains and vibrations, it does have its shortcomings. First, since the insert is not a single continuous piece, installation can sometimes be slightly challenging compared to solid-body inserts. In addition, the discontinuous nature of the insert means that using an anaerobic thread locker means that both the insert and the fastener screwed into the insert are both going to get the thread locking compound whether that’s your intention or not. When a repair is being made on the vehicle, special attention must also be exercised to ensure that the locking tang doesn’t become a foreign object that falls into a critical area. Finally, the strength of a HeliCoil repair has been demonstrated to be less than a solid body insert. We hope to test a number of different thread repair insert designs in the future to give this a more definite number.

TIME-SERT Timed Solid Body Insert

While not as well known to the mainstream, TIME-SERT and the family of solid body inserts are many mechanics number one choice for thread repairs in high-preload fastener applications. This solid body thread repair solution uses a solid carbon steel or stainless-steel continuous insert that has the outside and inside threads in sync or (in time) with each other. By keeping the threads lined up on the inside and outside of the insert, the cross-sectional-area or wall thickness can remain consistent. The result is the ability to maximize strength for a given outside diameter of insert.

TIME-SERTs utilize the same drill and tap requirements as HeliCoils, as both use S.T.I. taps. Installation is similar, but there are a few key differences due to the difference in design. First, TIME-SERTs use an oversized locating flange on the top end of the insert. Hence a combination drill/counter-boring tool or a drill and separate counterbore must be used. This flange allows the thread to be repeatably located or positioned at a specific location (something that cannot be accomplished with a HeliCoil or Keenserts). Second, there is no tang for locking. TIME-SERTs use a special tool to lock the bottom threads into the threaded repair hole. Since TIME-SERTs are a solid body, thread locking compounds can be used on the inserts outer threads for additional strength without it affecting the inside threads and fastener.

While TIME-SERTs offer advantages over other thread repair insert designs, it also has a few drawbacks. First, it’s about twice the price of a HeliCoil per insert. Second, it must be installed directionally, as its flanged-design requires the non-flanged side to be threaded in first. On through holes, it can be used by installing it from either side. On blind holes that are not deep enough to provide enough room for threads and the flange, the flange side can be milled or filed off al later. However, if there were only one design of insert that we’d have access to for all thread repairs, we’d probably select TIME-SERTs. TIME-SERTs deliver the highest strength with the least amount of intrusiveness with regard to the size of the repair threads versus the original threads.

Keenserts Keyed Threaded Body Repair Inserts

Keenserts are very thick-walled, solid body thread repair inserts with retaining keys. While a Keensert looks to be very similar in design to a TIME-SERT, there are major differences. While both are solid body designs that are flanged (requiring counterboring for installation), Keenserts do not have the inner and outer threads “timed” with each other. In fact, a Keensert may have an outer thread with an entirely different thread pitch than the inner. Unlike thread-repair inserts that require S.T.I. taps, Keenserts utilize standard taps that are two steps larger than the original threaded hole. This means that an M14x1.5 tap would be used for a M10x1.25 thread repair insert. As such, the amount of material that must be drilled out and the size of the tap are both considerably larger than what is required for a HeliCoil or TIME-SERT repair.

For an M10x1.25 thread repair with a Keensert, a 12.5mm drill would be used for the M14x1.5 tap. The M14x1.5 tap would have a major diameter up to 14.4mm (an M10 repair with an S.T.I. tap would only have a 11.787mm major diameter). That’s a much bigger hole which can be beneficial in some application where severe corrosion is prevalent in the original threads. The Keensert would then be threaded into the drilled, tapped and cleaned hole with the Keensert installation tool. Thread locking compounds can be used due to its solid body design. Once set in position, the installation tool is then used to drive in the four key stakes into position. This provides the ultimate mechanical locking of the insert in place. Hence, this design sports exceptional resistance to vibration.

Now the downside. The massive outside diameter of the insert makes it unviable in applications where the material around the original threaded hole is limited, such as any “thin-wall” castings. Second, the cost for a standard-steel Keensert is about 4.5 times that of a HeliCoil and more than double the cost of a TIME-SERT. If you want a stainless-steel Keensert, the cost goes through the roof being about 25 times the cost of a HeliCoil.

BIG-SERT Thread Repair Insert

For applications that will benefit from a large-diameter insert or when you are replacing a standard TIME-SERT, TIME-SERT offers a line of oversized BIG-SERTs that are thicker wall and use an oversized S.T.I. tap. For example, an M12x1.25 S.T.I. tap would be used on an M10x1.25 BIG-SERT insert.

The Bottom Line

HeliCoil, TIME-SERT, Keensert or BIG-SERT? We’ve had the opportunity to use all three designs in real-world conditions. So which type of insert is best? The reality is that it all depends on the application and quality of the installation. We’ve never had any of these inserts fail on us. Are we lucky? Not really. Simply having the knowledge of the strengths and limitations of each design will allow you to select the right insert for the job. If you are not sure, call the manufacturer and ask.