
Text and Photos by Michael Ferrara // Illustrations by Paul Laguette
DSPORT Issue #166
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.).

This illustration displays the difference between TIME-SERT inserts which sync the outside and inside threads with each other versus Keenserts that do not sync outer and inner threads, which leads to inconsistent wall thickness and the need for a larger O.D. sizing of the insert.
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.

Once the hole has been drilled and tapped with the S.T.I. tap, 1) the TIME-SERT installation tool gets a drop of oil before 2) the TIME- SERT is threaded onto the tool. 3) Threadlocker can be applied on the outer threads and 4) inserted into the hole to be repaired. When the insert’s flange bottoms out 5) it also locks the bottom threads in place. 6) Remove the insert tool and the 7) TIME-SERT repaired hole is ready.
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.
To retain the insert, TIME- SERTs use a flange up top and a swedged fit at bottom, while Keenserts use two or four stakes.

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.
Big, Bigger, Biggest! Here is a comparison of a standard (left) M12x1.5 tap, a M12x1.5 S.T.I. tap (center) for TIME-SERT and HeliCoil inserts and a M16 tap for Keenserts (right).
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.
The TIME-SERT Universal Head Bolt Thread Repair Kit (for first time repairs) come with fixture plate, alignment tool, a drill/counterbore bit, tap, installation driver, metal rule, stop collar, lube, threadlocker and more.
Keenserts: Keyed threaded body insert
+ Thick-wall design delivers highest strength for a given bolt diameter + Uses conventional, easy-to-source taps + Solid design provides easy installation + Solid design allows use of thread locking sealer on outer threads + Flanged design allows for positive location of the threads into the object + Positive locking design with four key stakes provides maximum vibration resistance – Makes the largest hole and requires largest tap diameter versus final bolt diameter – Over 5x cost of HeliCoil, Over 2x cost of TIME-SERT – Availability: Hardest to source, limited distributorsTIME-SERT: Timed threaded body insert
+ Thin-wall design delivers highest strength for a given repair-tap diameter + Smaller hole and tap requirements than “keyed” style insert + Solid design provides easy installation + Solid design allows use of thread locking sealer on outer threads + No weak spots…consistent wall thickness due to “timed” design + Flanged design allows for positive location of the threads into the object + Self-locking design – Higher cost than coiled thread inserts – Availability: Not available at most automotive retailers – Uses harder-to-source S.T.I.-sized tapsHeliCoil: Helical Coiled Thread Repair Inserts
+ Helical coiled wire design results in smaller hole and tap requirements than “keyed” style insert + Most readily available thread repair solution + Lowest cost thread repair solution + Can install from backside on through holes – Cannot use thread locking compounds on coiled insert without affecting inner threads too – Uses harder-to-source S.T.I.-sized taps – Tangs can become FOD – Only held in by friction
4 thoughts on “Three Ways to Fix a Damaged Thread | Insert Tech 101”
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I have a evo X and want to time insert my block. im running high boost at 45psi. what kit can i use. or wan t can i use
Did you read the article, Alfredo?
Just what I needed to read, thanks very much. I’m going to get an M12 x 1.75 tap for the head bolt in my QR25DE Sentra. Time sert all the way!
I HAVE A 96 INFINITI I30 AND THE 3 BOLTS IN THE BACK OF THE LOWER CONTROL ARM IS STRIPPED. I KNOW ITS DEALING WITH THE SUSPENSION SO WHAT SHOULD I USE? THE BOLT SIZE IS A 22 MM HEAD BOLT